You could:
1. make a connection between per project and equity allocation - so each time they complete a project, they get a small % of equity. Research has shown that people prefer small payouts than one big payout at the end (ppl can't postpone their desires...).
2. Keep them involved in the decision loop and share with them small successes. Make them feel part of the company/business.
3. Give out additional 'bonuses' for projects done well or for milestones achieved by the company.
4. If you have the capital/money to support it, then make it clear that they can exchange their equity for money - if a company is willing to buy back it's equity, this shows that the equity has value - which will make them want to keep it. The only challenge with this last option is that if you do buy back shares, it creates a tax event for both sides which isn't always advised.
There are a few additional options.
I've successfully helped over 300 entrepreneurs and would be happy to help you. After scheduling a call, be sure to please send me some background information so that I can prepare before - thus giving you maximum value for your money during the call itself.