This is where tubing benders come in. The common theme among all tubing benders is to maintain the round cross-section of the tubing as it's being bent. The springs pictured are one of the cheapest, simplest and easiest to understand. They are inserted into (and sometimes also around) the tubing and provide support that keeps the top and bottom of the bend from bowing out and the inside of the bend from collapsing. It is similar in concept to what's known as mandrel bending. These bending springs are, however, only suitable for relatively small diameter soft tubing.