As much as necessary to avoid any confusion or ambiguity.
The answers below are pertinent but there are many, many other considerations. Such an agreement will have to be tailored to your particular situation. I strongly recommend that you consult with an experienced IP attorneyand do not attempt to do this yourself. Furgang & Adwar, L.L.P.
I will add some items for the other side. If you are the corporation, you should always have a clear separation clause to make breaking the agreement clean and simple. It is often better to give more in a break-up to save the time and effort and lack of clarity that otherwise results.
You want to specify long term goals for the agreement. Are you expecting this to become an employment agreement once financing occurs? Is this a one up? How long will it go on. This is important because equity is best traded for future benefit as opposed to past benefit. What happens if the goals are reached but the person refuses to come on board? These issues and clauses should all be addressed/incorporated.
A few things are part of most sweat equity agreements.
1) Vesting period - How long do you have to work before your equity is vested and you become part of the ownership of the company? Define this up front in writing so there is no ambiguity.
2) Performance Criteria - It's not always possible, but try to be specific about what you are expected to do in this role. In startup / equity situations, it's easy to fall into roles where you are expected to wear a couple hats. Your efforts to assist the company to grow may backfire if you get stretched too thin. Well-defined performance criteria can eliminate ambiguity and provide you with a detailed plan for how to achieve success.
3) Nature of the equity - Are you working for stock? If so, specify the number of shares you will receive as well as the type of shares. Preferred shares are almost always what you want to receive.
4) Formalize the nature of the entity - sometimes, you are working for stock in an existing business, sometimes not. If you are working for an ownership stake in an LLC, make sure your agreement is explicit about being added to the list of partners and that a formal operating agreement is in place. The structure of these entities can change based on a simple majority vote, meaning you could be given equity then simply voted out by other partners. If it is a c-corp or an s-corp, things are a little more structured.
5) Separation Criteria - lay out the specific conditions under which you can be removed from your role with the organization. You do not want to be in a position where you are fired a couple days before you are to receive equity in the business. Be clear about the conditions under which the agreement can be broken and do everything you can not to run afoul of them.