1 - Do no try to mentor too many startups
Useless - I saw this at 500Startups. Too many people are mentoring too many startups. For the entrepreneur or the team you are mentoring the most important thing is bringing your experience to the table. Your specific experience in a specific field, most of the time. There are exceptions, but try to be the mentor of a few set of startups (2 to 4) and focus on them with clear goals also on your side.
2 - Real examples (don't be vague)
Many times after a mentor meeting, as an entrepreneur, you will feel totally lost. You have different mentors and everyone is just telling you different things. You are the one taking the final decision. In this case I usually rely on examples and experiences from those mentors: when you are trying to suggest an option, a specific path or a solution, bring a real example in your conversation. That will create trust and differentiate your opinion from thousands of other comments.
3 - Mentorship is a long path
The best mentors I had were "following" me. That's not having a conversation for an hour or just asking for a couple of suggestions and that's also why, as a mentor, you should be super clear on what you expect in return - it's just the best way to start that relationship.
Just another note.
Most of the people that used to be my mentors became very close friends. In most cases we used to do business together and the fact I was younger helped me getting the best out of that relationships. If you are doing a startup you should try to work with other entrepreneurs that love to be mentors or at least with people that have been in your position before - that will make all the difference.