Ok well all I can do is post what has worked for me..and from talking to a huge number of women online what DOES NOT work for women...

Now there are a massive number of web dating sites..some totally aimed at dating (faceparty, plentyoffish etc..) then the more community based sites (myspace, facebook etc..) doesnt really matter this works whatever and wherever the same way..

Approaching a woman online is not too dissimilar to RL - although of course you can happily hide behind your sceen to diminish your fear of failure.
Did you know that an averagely good looking woman on an averagely popular site will get around 100 NEW messages every day..not even counting the existing friends she has...she will bin a good 70-80 of those purely based on a dodgy username...

Oh and before you read the below..my username here I would NEVER use on a dating site....its an old nerdy one - but hey I'm amongst friends...

User name...Try to avoid overtly sexual usernames...hungbob...bigjim...Matt694U...NOOOOOOOOOO big big no no...99.9% of women will not even read a message from someone with a name like that (BIN)...also anything materialistic...loadedboy..(BIN).anything nerdy....PS3Guy ((BIN)...anything car related...porschedude..(BIN)..all wrong wrong wong !!!

As in all other situations be ambiguous and disarming...aim to have a username that begs further discussion...be silly with it if you like..stick some stuff in a random name generator on the web ...ElephantPainter....Stickdrawing....or failing that be honest and use where you live..LondonBob...USAJoe..MiamiGuy...these are all harmless and while may not instantly draw a woman into you will certainly not mean you get junked with all the others as soon as she logs on..so at least you are in with a (on average) now 20:1 shot of having your message read...more to follow...

Well put, I agree.

Your username should also match somewhat your age. Nobody is going to take you seriously if yours is something lame like "hotstud13".