Over at Galleycat they have some great advice from Allison Pang, a debut urban fantasy author, on how she secured an agent.
Know your market (ie, read what's out there and make sure you're pitching to the right people and that your vision is fresh).
Agents are not beta-readers. The manuscript that lands in an agent's inbox has to be the best it can be. Critically attack and improve your novel before you send it out.
Be positive. If you've prepared your MS and researched and polished your submission proposal to the nth degree, then you should have confidence in it and yourself. If you don't, why should an agent?