How should I negotiate for equity in a company?

I've been recruited to be the sole developer on a service based start up (phone based tech support). I'd be making the back end of their website and integrating it with a ticket system. This will probably end up with me having a heavy hand in work flow and other aspects of how the company runs. We've discussed hiring me at my hourly rate, but they've mentioned they don't have an income yet, and asked if I can take it easy on my estimates. I'm not worried about getting paid down the line, but I would like to negotiate for a piece of the company as I think I have more to offer than just being a programmer for hire.

As I see it. I've got a couple of bargaining chips. 1) I've got a good amount of acknowledged experience and knowledge in both fields (theirs and mine). 2) They would like more work out of me then they have ability to pay for. 3) I've got a heck of a lot more knowledge in leveraging mediums such as twitter, and blogs. 4) I would love to spend more time on the company then just coding a back end.

I'm also not sure exactly what I'm asking for and what's reasonable to ask for. I'd rather make money monthly from the business then when they might sell it down the line, but I don't even know if that's a smart. I could alternatively build the system and lease it to them, but then they have less incentive to work with me and I wouldn't have nearly as much of a stake.

I've never worked with a company where this seemed like a viable option, so I've never had a chance to negotiate. Any tips on how I should go about this and what I might ask for?