Before being tech people, they are just people.
How do you convince a technical cofounder to join your startup? The same way you convince anyone:
As any other person, a technical cofounder will only join your startup if they feel it is the right thing to do and that the risk of it not working out is low (also the payoff, compared to the risk, is high).
I believe there is one key to risk: money.
By far the largest risk to a startup is running out of money / not making enough money (not enough people want to buy). How do you solve that, since you certainly need the product to sell the product, right? WRONG. You can pre-sell your idea to early adopters. By doing this you prove there is a willing market looking to buy your soon-to-be product, thus reducing the perceived risk of joining your startup. Everyone wants to join a winning team, just look at how many people want to work for Google and Facebook.