What precautions do you take to really make sure you won't get pregnant when using condoms?

Some of you may have made the experience with bursted condoms, or you didn't put on the condom early enough?

I especially would like to focus on patting since I noticed that my BF's penis often leaks in the heat of foreplay. Should I take better precautions and put on a condom before forplay? (I am a little afraid that I might introduce sperm into my vagina when touchig his leaking penis, I guess you know what I mean...)