I have trouble playing Aatrox into a lot of top lane picks as well.
I don't claim to be an Aatrox master but here are some of my thoughts.
You should try to stack your passive as fast as you can from level one if you start with W. Autoattacking minions with Blood Price and Qing away to saftey if you can't seem to win a trade is probably a good idea as well. The bonus attack speed from passive once full or close to full will help immensly with your Csing and quick Autoattack harass.
You make sure you have your W passive ready for whichever.
His W is a large part of his strength, switch between them depending on the situation.
If faced with a counter-pick, or against a champion that you simply cannot facestomp early game, you will probably want to save Q for escapes; like if a Darius tries to start a fight while you're Csing, you Q away.
A lot of tips and tricks websites and comments from youtube to lolking to facebook mention that E is a great harass tool for match-ups you can't dive deep for. Like Versing and Irelia, or Riven, or Darius. Harass with E.
His Ult is pretty self explanatory and straightforward.
Just make sure you don't use it as a panic button, you'll lose the buff once you use your passive's active.
http://www.championselect.net/
This is a nice website to look for tips. Granted that all tips and comments are from players of all levels of skill. Double-check what they say.