Copper pipe is obviously hollow, and it is this feature that can give you problems when trying to bend it, i.e. it will crease.
The way around this problem is to use a 'bending spring'. The idea is that while you are bending the copper pipe (usually using your knee), the spring inside keeps the hollow piece constant.
Helpful Hint: When bending the pipe, remember to bend slightly further than your require. This enables you to bend back the other way, making it easier to retrieve the bending spring from the copper tube.