Chances are, if you're in straight and level flight and the load starts swinging, you're going too fast. First things first, slow down. Each load will have a speed that works best for it, so when accelerating with a new load, do so smoothly and pay attention to how it reacts.
Also, when making turns, keep them co-ordinated.
RH
PS Jetesoning a load is the LAST resort, for a swing in straight and level you shouldn't have to punch it.