That's a very broad question, but I'd say to look for the best players in your field. Of course, it sounds easier than it is, but then again, startups are not easy.

I don't believe it's a good idea to have many boards, the executive board should be the entity that should be advising you, you won't have time for much more. Also, remember to keep the number of the board members controlled. Ideally, if you're at an early stage, three members are more than enough (one seat for the Founder/CEO, one investor, and one independent expert... And if none of the investors are asking for a seat, then two experts).

Your board will be growing with time, so you don't want to have a board with 15 members by the time you get to series B.