The question is about how attract and what to offer to people who you want to be behind your start-up? It's easier with developers and other positions where you can translate their hours to pay at a defined rate, with industry experts it's probably different.
For example, there's a guy who I want to approach and he's an expert in affiliate marketing and merchant data feeds, but I have no idea what I can offer in a start-up that has nothing except web site in development and some interest from investors that might not immediately translate into funding.
Start by offering him a smile and a firm handshake :) Maybe start small and then offer him more of a role in the Start up later on.
If you don't know him, find someone who does that can make an introduction, or make an introduction yourself saying you have a mutual friend. Invite him to lunch, to a drink, or offer to stop by his office. Say something like, "Would you mind if I dropped by your office for 15 minutes to introduce myself... I'm working on a project that I know your brief advice would be greatly valued?"
Also, if you don't have anything specific to ask, be open with him, and say something along the lines of "hey, I'm just wanting to build a rapport with you, so as to have a chance to call on you for advice when I'm further along with this project."
From there, maybe follow up, "Hey, we've launched our website, and I was wondering if I could take you to lunch to get your advice on a few specific things we are working on?"
And being thankful and appreciative for other's time is also a good way to open doors in the future (thank you notes can go a long ways).