@Shannon H., probably the most important things to do when writing your own lease, is making sure it is compliant with all landlord/tenant laws, and that you are following any local guidelines specific to your location. You also want to make sure you are properly using legal terms in the contract. Once you've written your lease, you can always run it by an attorney for a fraction of what it would cost to purchase one from them.
Now back to your original question, the list of clauses that every lease should have is very long, and they're all equally important. I would look for examples in the File Place, googling Lease Agreements, etc., and look them all over. Definitely include the things you consistently see on all of the examples, and pick and choose what you may or may not want from each one (some examples may have small things others don't).
The one thing that has provided me with a feeling of safety (and this doesn't apply to every landlord, I use it because so far my tenants have all been military), is making sure there's a clause where the tenant provides the information of their 1st Sergeant and/or Commander, and gives me permission to contact them should any issues arise. The military makes sure it's members follow their legal and financial obligations, so by getting the problem solved by their leadership, I am avoiding having to go to the courthouse, accruing any legal fees, etc etc.
Allan