1) Industry expertise (for strategy, GTM, business model, etc)
2) Experience scaling a startup (for helping with operational issues, user acquisition, etc)
3) Has a strong network of contacts (for business development & partnerships, fundraising, hiring, etc)
4) Willing to commit time and effort to help
5) Someone you get along with
They do not have to hit all 5. You can have 3 advisors, for each of 1-3 above.