Contracts for professional services typically include:
1) Scope of Work -- a description of the work that you will be delivering, usually expressed as a list of deliverables. Also important in this section is a list of all things that are explicitly *excluded* from your scope of work.
2) Acceptance Criteria -- What criteria will the client be using to accept or reject your work? Acceptance criteria is defined per deliverable. This part of the contract is usually the most difficult to pin down because it forces you and your client to be explicit. However, the amount of work you put into this section prior to the project is proportional to the heartache that you will be spared later. Don't settle for criteria that are subjective because that defeats the purpose of having acceptance criteria to start with.
3) Schedule -- How long will it take you to complete your work? You can provide a schedule either by listing explicit tasks and time frames, or by simply indicating milestones and target dates for each milestone.
4) Client Responsibilities -- If your ability to make progress on your work is dependent on the client doing and/or providing something, you'll need to list these client responsibilities explicitly. That way, you'll have a right to ask for these things during the project and the client will have time to prepare and get you the things and people that you need.
5) Cost and Payment Terms -- how much you are charging for your service, and when do you get paid? Is there a lump-sum fee at the start? Is it a staggered payment scheme? Or is it based on you achieving a milestone? All this needs to be explicitly stated so there are no surprises later. If you will be charging for materials used during the project, you should also list such materials and provide indicative costs. Also include in this section anything to do with taxes. It may also help to include a clause that stipulates interest fees if your client is late with payments.
6) Ownership -- if you are using templates or other proprietary material to do your work, you may want to include an ownership section in the contract so there is no implied transfer of ownership of such material. Better yet, explicitly list the items that will become the property of the client upon the successful completion of your work. That way, anything that's not listed remains your property.
7) Post-Project Support -- what kind of support is the client entitled to after your design project? Can they call on you to tweak your design for another platform, and if yes, what kind of rates will you be charging?
8) General Contract Clauses -- other standard contract clauses include (a) clauses that govern how contractual disputes will be resolved; (b) clauses that limit your liability to a specific value; and (c) clauses that stipulate the applicable law -- e.g., which state's or country's law will be applicable. It may well be worth your while to consult a lawyer to get these other general contract clauses written up properly -- it's a one-time investment that you'll be using in all future contracts.
Finally, a disclaimer -- I'm not a lawyer so all this is just stuff that I've learned along the way. I also don't make any claims as to the completeness of the list above. But I hope it helps.