Well, of course figure out how much power you'll need, which mostly depends on your gfx card/cards...
Also modular or non-modular... modular means you can disconnect unneeded cables.
Efficiency, if that is important to you, look for an 80+ unit, with bronze, silver, and gold ratings.
Read buyers reviews, e.g. @ newegg.com

Hope that helps, and good luck!

As an example, I bought a Corsair HX750W modular PSU, with very high efficiency (>90%) 750W being more than enough for my needs. ~$200