If you know the direction the bend will go as you wire, bend the branch, at least partially into position, as you apply the wire. This will keep it tighter. WIre should end up on the OUTSIDE of the sharpest or apex of the bend.
Second tip, hard to explain. If you applied the wire clockwise, and bending loosens the wire, you should have wired counter-clockwise. (and the reverse too). Try removing the wire and changing to the reverse rotation. Then the wire should tighten as you bend. Take a few branches from landscape trees in the yard, and try it out. It is hard to visualize from a typed description. It does matter whether you wire clockwise or counterclockwise. After some practice you will see which direction to wrap just looking at the direction of the bend. Wire should be on the OUTSIDE of the sharpest point of the bend.