One of the things I love about bootstrapping is how creative you have to get. Get creative about your compensation. Whatever provides value to them. Experience, expanding their network, or just a good time. Things that might normally be benefits. You're looking for some way to motivate them to spend their time and energy on you, and not somewhere else. Just don't defraud people.
Once you've exhausted creative compensation, you've got cash, equity and convertible debt.
<strong>Cash:</strong> It sounds like you can't afford a cash salary, but try to pay <em>something</em> if you can. I think it's just easier to build trust that way
<strong>Equity:</strong> I would consider this last. The earlier your company is, the harder it is to value your equity. Since you're not adding a partner, I would recommend paying out shares on the same schedule that you would pay a salary. Monthly or bi-weekly or something like that. Then they only get what they work for.
<strong>Convertible Debt:</strong> This removes the valuation guessing problem. You figure out what you would pay them if you could, taking those creative compensation ideas into account, and make up for what you can't pay them with convertible debt. There are good explanations out there for how this works, so I won't go into detail here. I would start with this: http://www.foundrygroup.com/atvc/convertible-debt-series/