I'm sorry about your chemical pregnancies.

Often times, there is nothing that can be done to prevent a chemical, however since you've had two already, you might need your progesterone checked. If you find out that you are pregnant, I would call the doctor immediately. Does your doctor know your history with chemicals? If so, he or she may be willing to call you in some progesterone and then do a blood draw to check your levels. But with that in mind, the sooner you know, the better.
If it turns out that you're not pregnant this month, I would suggest ordering some "Internet cheapies." That way you can always have a whole stash of tests on hand and you won't have to worry about driving into town to buy more. They are really cheap...like 20 to 50 cents each.
Good luck! Please keep us updated.