A couple of thoughts:
1. Make sure you are aligned on the vision or the big "why". (Another entrepreneur had a pre-Facebook social network turned into a cable set top box, get my drift?)
2. Know the type of people you want to invest, and then qualify them against that. (Remember it's like being married, and it's not all about the money).
3. Do they bring domain expertise? Again, stupid money doesn't much help.
4. Talk to other startups they've invested in. What are they like? What has the experience been?
5. Be aligned on the exit.
6. Have clear boundaries about how much involvement in the day-to-day you want (or don't want)
7. What is their attitude about valuations and terms?
I guess all that is a ... start! Good luck! Let me know how it goes,