Make sure you are leaving that little bit of empty room at the very tip of the condom to give the semen room to collect there.
If you use condoms with spermicidal lubricant, that will offer a bit of extra protection in case of leaks and spills. There is also extra spermicide that you can buy, but you might have to be careful about that. Some things don't get along with latex and can make it break easier, so sometimes you have to be careful about what to use with condoms. I am not sure if there are any spermicides that cannot be used with condoms, but there are lubricants that can't be used with condoms. You could always check what's available at the store and ask you doctor.
If you have a regular partner, you could always talk to her about going on another form of birth control as well.