Stabilizers

You can try adding a set of jack in front of the axles. Get a set from a junk yard or cheap Harbor Freight ones and bolt them on. You can also try a set of Stack Jacks if you want something more portable. On the longer trailers this works really well to take out that mid trailer bounce.

Also make sure you're chocked very well. Some of the movement you may be feeling maybe coming from the tires rocking back and forth ever so slightly. Set the front chocks, pull forward slightly, then set the rear chocks. Some use X-chocks to accomplish this.