There's no easy way but I have found that one factor is critical - that YOU believe in your business and have a vision for where you know it's headed.
Think about it. You are trying to literally "sell" someone to become a part of your team that has nothing but an idea and you're telling them that they can take part in making it a reality.
Here's what's going on in their mind. Do I trust this person to lead the team? Do I think this person has the leadership abilities to exploit the business opportunity? Will other people also want to help out and join the team? Basically, they are evaluating YOUR ability to make it happen.
I can't stress hard enough that you should give them the hard facts up front. You won't get paid a large salary (I'm not even getting paid!) If you expect to help the team succeed you will put tears, sweat (maybe even blood...who knows...maybe you get a paper cut) into building the business. You WILL (and should) lose sleep over it. There is no ONE way to reach the solution (or they may even be multiple solutions to the business problem your team aims to solve). So be flexible, be ready to put in some long hours, and hopefully your new team member will enjoy the ride.
What can you promise them? That they will learn 10 times as fast compared to them sitting in a cubicle somewhere in a large corporation and that those skills will one day be used to build their OWN business. Don't promise a large salary, a ton of equity and perks - because this is NOT what motivates people to work for a start up (of course it plays a part, but the intangibles account for the drive and passion that people feel for building a business).
Good people will realize all of this and stick through it all with the team. It takes dedication, perseverance and believing in the leaders' ability and the vision of the business.
"All things go to those who go after them" - Rob Estes
So dream big and go build not just a good team, but a great team.