What could be causing my car not to start when the battery isnt dead?

First thing in the morning my car starts fine but if I go somewhere and run into a store or appt when I come back out my car won't start right away. It first started doing this sporadically but seems to becoming a regular problem. The battery isn't dead, lights, radio and air conditioner will come on and I can even roll my windows down or up but my car just clicks and won't turn over. And this doesn't even happen everytime, sometimes it cranks fine. But on the occassion that it won't start I just have to be patient and wait and try to crank it every 5 mins or so and eventually it will start. ILve poured coke on the battery pegs just to make sure their wasn't a connection problem but that didn't help. I just really don't want to spend a fortune to take it somewhere if it could be something easy to fix.