I've served as an advisor to a number of startups, both in formal accelerator / incubator programs, and on the advisory boards you are describing (not to mention the free advice for friends and family).

It sounds like you've already decided that you want to do it, and you're just looking for potential red flags or pitfalls, so I'll skip the pitch on how rewarding it can be, even if it doesn't "pay off".

Factors to consider:
1. Companies pivot. Founders rarely do. Focus more on the who than the what, because the idea and model might change drastically over time. It's important to like and trust the people you'll be working with.

2. How do you fit with the rest of the advisory board, and is that important? Some entrepreneurs like their advisors to meet and work as a group, while others deal with their advisors very independently. Know what style and what type of team you're signing on to.

Questions to Ask:
1. "What do you expect from an advisor?" You'll get a lot of different answers from different entrepreneurs - make sure your expectations are aligned. Talk about the important things like honesty and accountability, as well as the potential frustrations like responsiveness and scheduling expectations.

2. "What skills / knowledge / experience are you looking for in me specifically?" I might think I have a great marketing mind, but if the board of advisors already has a marketing expert, they may only be looking for my technical knowledge or relationships. Make sure you don't think you're bringing something to the table that isn't what they're looking for.

Lastly, in terms of the deal structure, be honest if they make an offer that you feel undervalues your time and / or experience. Steer the conversation toward opportunity cost (If I commit to you, I have to say "no" to other things that require my time) - that simultaneously demonstrates that you know your value and that you intend to abide by your commitments.