2. Who is responsible for paying for what. As part of your contract you want to specify whether or not the overall price paid includes materials or is just labor. Are they responsible for purchasing the items used? Or are you? What if they break something? Do they have to pay for it? Or do you? This all needs to be specified.
3. A specification of the period of performance. It is important for the sub to give you a time line, they should tell you how long the job will take them. You then need to include this in the contract. You need to include a date and time they will start. What time each day they are expected to be on site, and when the job must be finished by in order for them to get paid.
4. Where work will be preformed. You will want to specify where it is that the sub contractor will do work. Include the full address.
5. Who pays for off schedule work. If the work takes longer, or causes delays in other people's job, who is going to be responsible for these costs? What is typically best is to attach a financial penalty to the contract for each day late. If they say things will be done in 5 days, and it take 8, you should be able to take $50 a day off the price you originally agreed on. This will motivate them to get things done on time, and protect you if they don't.
6. Any exceptions. You will want to include in your legal contract any exceptions to stated things, such as if an earthquake happens and breaks all windows, they do not have to pay for them, or if a hurricane comes through and rips the roof off, etc.
7. Who is responsible for direct costs, such as travel. If they are traveling to your site everyday, are you responsible for their travel costs? What about their lunch? Be specific about what is paid for by you and what is going to be their responsibility.
8. Have a clause about who will order, pick up, and pay for materials. This is important because if you are supposed to, and you don't and that is why they can't finish on time, that is your problem. But if they are supposed to and don't, then the excuse of not having the materials does not fly.
9. What things are included in price/bid. Does their bid price include just labor? Does it include materials? Does it include some materials but not others? What about clean up?
10. Safety. You want to have a clause about safety that requires them to wear proper attire, drink water to stay hydrated if working in sun, wear hard hats, etc. this clause protects you should accident happen because of their neglect. It is also important to include safety of materials, meaning if things get stolen, they are responsible to replace them. (this deters them from stealing materials, and reminds them to lock up and secure the site).

There is probably more you should have, but always have an attorney review your contracts to make sure there are no loop holes and that you are covered legally.