Deciding Which Play To Make
With each turn, you want to make the best play given the circumstances. There will usually be a number of different words you can play, you have the option to trade in between one and all of your letters, or you can pass. The first thing to do is to try to find the biggest play available. If it's a really big one, say at least 40 points, you're done. Usually that's not the case, and instead you'll have a handful of similar plays in the same general scoring range. Most players will play the one that scores the most points, but that's not always the best play. Offhand, I'd guess that I play my highest-scoring word around 70% of the time. Score is the most important factor, but there are other factors to consider: what letters you're left with, defense, buildability, and how well a play is suited to your remaining letters.
Scoring
First you need to figure out what sorts of words you can make with the letters you have. Look for sets of letters that appear in a lot of words, such as ED, ER, ING, TION, IGHT, FUL, PRE, etc. The first group of words to find are ones that can be made from your letters. To play one of those, you'll need to be able to form an additional perpendicular word. The other group of words is those that can be made using letters (usually just one) currently on the board. Since you won't be making multiple words, it's more important for it to score well.
The key to piling up points is to use the multipliers effectively. The board contains double- and triple-letter multipliers (DL and TL), and double- and triple-word multipliers (DW and TW), arranged in a well-thought-out pattern. The layout of the board makes for more interesting gameplay than does Scrabble's layout, in my opinion. There is another implicit multiplier: forming multiple words. Often a very good play can be made by laying a word alongside an existing word. Finally, there is a 35-point bonus for using all seven of your letters (again, a good choice compared to Scrabble's 50-point bonus, which is too dominant).
The best way to use these multipliers is with high-value letters. That's pretty obvious, but it's very important. The higher a letter's value, the more important it is to get it counted at least twice and hopefully more. The chance you can do that depends on the letter's usability, which doesn't always map directly to its point value. There are some sweet spots and some dry areas in the WWF alphabet. Some letters I love, some I hate. Here is a summary of the point values:
0 points: blank
1 point: A E I O R S T
2 points: D L N U
3 points: G H Y
4 points: B C F M P W
5 points: K V
8 points: X
10 points: J Q Z
The most powerful letters are the blank and the S. Those should be doing you some serious good when you play them. Usually I want the S to be gaining me at least ten points when I play it, and the blank fifteen or more. That's not always possible, and it incurs the downside of having tiles you're not willing to use cluttering up your rack.
My favorite letters (aside from the obvious two above) are H B F M P and W, which I feel hit the sweet spot of points and usability. They appear in a lot of two-letter words and generally work well with other letters and most vowels. For those, I hope to get at least 3X value. My least favorite letters are C V and K. The extra point for V and K does not make up for their unwieldiness, and C is just as bad while being worth less. The main hindrance for C and V is that they don't appear in any two-letter words, so playing them in both directions is difficult. I don't mind getting just 2X for those cruddy letters, or even dumping them for 1X (face value).
Down on the low end, the one-pointers are used mostly to leverage bigger words and provide opportunities. They are also useful in pursuing the 35-point bingo. I don't mind getting 1X for them. A corollary to that is that I generally avoid putting a one-pointer on a 2L or 3L spot. The two- and three-point letters fall in the middle: I hope for 2X or 3X respectively (same as the point value, semi-coincidentally). Of course, a lot depends on how much flexibility your rack gives you, and these suggestions are most useful when you have lots of options.
The key to really high scores is using more than one multiplier at a time. The layout of the board provides for several specific high-yield plays:
TL + TL (underrated - can be close to value of TW)
DW + DL
DW + TL
DW + DW (essentially a QW - quadruple word - sweet!)
TW + TL
If you can get two-way action on one of the multipliers, that's even better. The most common placement for a really big score is along an edge with a high-value letter going both ways on the TL, within a word that covers the TW.
If you can play a bingo (all seven letters), you should, even if it's somehow not your highest-scoring play, because bingos are cool.
The holy grail of multipliers is an eight-letter word spanning DW + TL + TW. I've only ever been able to do that twice (DIABETIC and ALLSPICE if you're curious, or even if you're not now that you've read it anyway).
You And Your Rack
Unless you play a bingo (all seven letters), you'll be left with one or more letters in your rack when you play, letters you'll have to work with on your next turn. So you should do what you can to make sure your next turn will be enjoyable. If possible, I try to play so that I'm left with at least one vowel and one consonant. Never trust the game to give you good letters to replace the ones you played. Plan for bad letters, or all vowels or all consonants. If you can dump a letter you don't like (or one that's less useful given the board), do that even if you get a point or two less.
Defense
Defense is the rainy day of word games. As in any sport, it's undervalued because it's not nearly as fun as offense. In WWF, I play only moderate defense. To give an idea of the range, a very defensive play would be playing IT on a TW for six points, so that your opponent (who probably has much better options) cannot use that spot to make a big play. Non-defensive plays are those that open up the premium scoring areas (such as the TL + TW combo). If I'm going to open one of those up, I want it to be with a decent play - that way we're both scoring some points. The better your rack, the less defense you need to play, since offense tends to be contagious and you have a good chance of keeping up. It's more fun to lose 390-412 than it is to win 220-215.
Buildability And Suitability
Buildable words are ones that can add letters to the front and/or back to form new words. For example, RACE is a very buildable word, RICE is okay, and RICH not so much. Another form of buildability is parallel play: words laid alongside that form multiple two-letter words. The words above don't do so well there since each has a C, for which there are no two-letter words. Since there are a ton of two-letter words, most words have good parallel buildability.
Suitability is a bit harder to explain. It's mostly intuitive. One way I think of it as how fertile a particular play is with regard to the board and my remaining letters. If the letters I have left can be added to or played alongside the word, cool. You'll notice as you play that certain types of words tend to get grouped together, depending on things like vowel/consonant patterns.
Tricks
Leave an extender in your rack: If you have two plays that differ by a starting or ending letter, leave that letter in your rack so you can use it to make a two-way play later. For example, if you can play MATH, consider playing MAT and using the H to turn it into MATH later while you make a new word with the H. This can be a very fruitful strategy.
If a really good place to play is open (eg a wide-open TW) and you can't use it, open up another one. That way there will be at least one available on your next turn. That's more fun than just trying to block it with a defensive play.
Sometimes you may want to know if something is a word without having to play it, as you consider your options. The way to do that is to play it while you also make a crossing nonsense word like VM or DDC. WWF will tell you all the words that were invalid, so you'll know if the one you were testing is good.
First Turn
Generally, if you can get to the DW and score decent points, that's your play. Otherwise, all options are open. There's no need to score a lot, so a single-digit score is fine. Dump letters you don't like. Play something you can extend. This is also the best time to swap if you feel like it. The only real reason to pass is if you have seven letters of an eight-letter word, and you want to see if your opponent plays that missing letter.
When To Swap
I prefer dumping bad letters to swapping, but if I think it will take me more than two turns to get rid of the crap and work my way to a solid rack, I'll swap. The only tiles I never swap are blank and S. If I have one or more of those and the rest of my rack (which is all I'll be using if there are no good spots for the blank or the S) is useless, I'll swap to see if I can't get some good fodder to combine with the power tiles.
