Welcome to the forum!
I'm working on the solution to the same challenge.
As Mike linked, the ice/fish house industry has a pretty good thing going with regard to having solved the problem (though likely without knowing that we have a problem they solved...)
The majority of their systems use a simple ratcheting cable lift to allow one end of the trailers leaf springs to raise/lower a couple feet to lower the trailer onto the ground.
They also use a pivoting tongue so the front also lowers.
The fancier models use a hydraulic system connected to trailing arms to which are attached to torsion axles.
If you are good at fabrication and welding, you can build your own.
If not, the going prices for these setups on trailer frames (no body) run $1500 - $4000 depending on size and options.
I have seen some air bag models, but they are home built.
http://www.youtube.com/watch?v=N_wnoqqH ... re=related