I have been playing for months (before the XBLA version) and I am better than most players, however, I am nowhere near as good as the best players. However, I am still able to regularly get into the top 10%, I have over 200 top 10 finishes, and a handful of rank 1 finishes. I think it is very possible for all players to unlock most of the Achievements with practice.
Clean your screen for fast, smooth swiping!
If you play very late at night (US time zones), there will be fewer people playing. If you're already pretty good, this can help make a difference in getting in the top 10% or top 10 ranks. I do get in the top 10 every now and then, but the handful of times that I've gotten the number 1 rank have all been late at night.
You can also try playing in another language (only English and Spanish are available right now), as a lot fewer people will be playing. I got 6th place in a Spanish round even though I started mid-round, guessed a whole lot, and was generally terrible (though I actually do know a little Spanish).
Take a quick moment to survey the board. You don't want spend the whole time focusing on one corner of the board because you find a lot of (seemingly) good words in one corner only to realize at the end that you could have scored a lot more points had you noticed sooner that there was a J or a set of suffix letters ('ING', for example) that you didn't properly take advantage of.
Longer words are worth a lot more points due to the effect of multipliers:
3-letter words -- 1x points
4 or 5 letters -- 1.5x
6 or 7 letters -- 2x
8+ letters -- 3x
It is possible to score very well by finding a lot of short words very fast, but in general it's better to focus on finding longer words. Try to focus on longer words first, and then go for short words like crazy towards the end of the round if you start to run out of steam. For instance, if you find 'pinning' and 'penning', you might also be tempted to go for 'pin', 'pins', 'pen', and 'pens', but generally, the time is better spent finding larger words. Don't waste time on low scoring words unless you are stuck. Most 3-letter words on a board are only going to get you 5-8 points. A good 6-letter word can easily be worth three or four times that. It's much more time economical to go for longer words, even if you have to spend some of that time thinking.
I still score very well even when I let my finger sit idle for a few seconds while I look for words. Again, longer words are better. However, obviously there must be some balance. It is obviously better to spend time scoring than not scoring. So if you find that you are spending a lot of time not scoring while you look for long words, then you should start going after shorter words. It takes some practice and experience to figure out your style and level of skill. Once you get a feel for it, you will know how to transition between hunting for higher scoring words and simply getting any word you can find.
Obviously, you should also focus on bonus score Digram and Theme words as you would longer words.
In the same vein, if you see high-scoring tiles (J, X, Q, etc) focus on using those.
Know the language! Speaking of English specifically, learn to recognize prefixes and suffixes and how they combine in different situations. This can easily help you turn a few smaller words into a whole family of long words.
Remember that many base words in English can take almost all forms (nouns, verbs, adjectives, and adverbs). For instance, if you only think of 'showing' as a verb, you might be happy with that, as it's a pretty good 7-letter word. However 'showing' is also a noun. This means you might also be able to score 'showings' (plural); that makes it an 8-letter word with a 3x multiplier. So of course, look for prefixes (examples: re-, un-, en-, ex-, de-) and suffixes (examples: -s, -es, -ed, -er, -ier, -y, -ly, -ing, -tion) and think of all forms of words (this will help you recognize that you might be able to pluralize it or -tion it or -er it, etc etc).
Recognize that some base words change when they are conjugated. For instance, if you see the letters R-A-T-E and also D, S, I, N, and G close together. You might think about the word 'rate' and then score 'rate', 'rated', 'rates', 'rating', and 'ratings'. That's good. However, what if the 'E' wasn't there? A lot of people at first will only see the word 'rat' (and 'art' and 'tar') but then they'll miss big scoring words 'rating' and 'ratings' because they're not thinking about the word 'rate'. So just try to learn to recognize that sometimes there can be good, high scoring words present even when you don't see the base word in its normal form.
Again, talking about English, look for common constructs in words, especially ones that involve valuable letters or a lot of letters. For instance, if you see a D, G, and an E, note that G is a valuable letter and that there are many words that can be constructed with these parts (adge, edge, idge, odge, udge), and for example: badge, badges, badger, badgers. Another example of common valuable constructs: CK, TCH (itch, atch, otch).
My final piece of advice is something that I am actually not very good at: look for compound words. This is something that I have the most trouble with as it involves either intuitively being aware of them remembering words you already found and thinking of whether they can combine with other words. As an example, FIREPLACES is one of the highest scoring words I have ever found (due to a theme bonus, I think). However, I rarely find words like this, because separately, the word 'fire' and the word 'place' aren't really related, and I just tend not to see this kind of connection most of the time. However, this can be a powerful way to score big points because all three words (fire, place, and fireplace) are very common. Separately, they're not that valuable, but together, the two common words easily turn into one of the highest scoring words on the board. It's funny, I usually tend to find more obscure words and miss common, but lengthy compound words like this.