I have a flat bed trailer and I want to know if there's anything I can do.?

Get both tires balanced. The weight holds the wheels on the pavement but when its empty there is nothing holding it down, the slightest imbalance causes the wheel to bounce more with increased speed. Even with perfectly balanced wheels a trailer that is too light will bounce a little. Try putting some sandbags on the corners of the trailer .

That is unavoidable with spring suspensions. The suspension is designed to carry a certain amount of weight (not that it is smooth then, but it's much better), if you are much lighter than that you got bounce! The suspension is pushing up on the trailer/down on the road (especially those bumps all over the place) as if it was fully loaded. That's the reason they came out with air ride (airbags) suspensions - smoother riding loaded (varying weights) or empty. Virtually no commercial trailers have anything but air ride anymore.

You didn't mention the size of your trailer but there are air ride suspensions available for most any - it might cost more than you want to spend.