What has always worked for me, was to park the tractor with the 3pt hitch lowered. When i use the tractor, i close the 3pt valve under the seat, then i raise my ballast. It rarely drops much at all. If it does drop a little, i just pull the lift lever and raise it back up. I dont think i have to raise more than twice in one day while using the tractor. Some do leak down faster than others, but mine stays put pretty good.
If it is a ballast box (or any other implement) you dont plan on removing for long periods of time, chain it or block it in the up position. I have a custom made locking bar i made for mine, but i dont use it now. It got to be a hassle because i change out implements so often.