I'm sorry to start with a lecture but these irritating preliminary steps could potentially save you a lot of time and effort.
Before you start it is best to check
1)The condition of your tyres – bulging side walls for example could mess up your measurements
2)Tyre pressures need to be correct
3)The car should not be overladen – take out all of that stuff in your trunk!
4)The fuel tank should not be empty – it is best to do this with a nearly if not a full tank
5)The condition of your suspension components should be good
6)The car should be level AND on a level surface (this was difficulty for me)
7)Your wheel bearings should be correctly adjusted and your wheel bolts need to be tight.
8)Your wheels must not be damaged
I think these are the main things to check. Even so read through this thread for a better overview:-
wheel alignment DIY
(Please do this: undoubtedly whunter is more experienced than I am)