Jeff Nolan has a great post entitled Pick Your VC Carefully. A must read for any entrepreneur (or VC for that matter).
It’s worth the full read, but his main points are:
1. Pick the right type of investor — big, small and specialized, financial, corporate.
2. Check their references!
3. Make sure you understand how much pull your investor has within his or her firm.
All good advice, some overlap with my posting on How to Negotiate a Term Sheet with a VC. My only addition beyond what’s already in that post is that if you’re adding a new investor into a syndicate, make sure you have your existing investors spend time speaking with the prospective investor, both with you present and without you present. You’ll learn a lot about your future board dynamics that way.