To maintain the circular section when bending a copper pipe use a bending spring – a specialized tool which can be hired from a do-it-yourself shop. Make sure that the spring is the same size as the internal diameter of the pipe.
If, however, a bending spring is not available, an acceptable alternative is to plug one end of the pipe and pack it tightly with damp sand. When the pipe is full, plug the other end. The pipe can now be bent without any risk of it buckling or crimping.