Barry,
I do it to my O'Day 222 with four sailboat jack stands, a hydraulic jack, and some 4' lengths of 6X6s and old power company cross arms. I place the four sailboat jacks near the stern and crank them up. Then I use short doubled up planks on the trailer under the forward portion of the hull to set my hydraulic jack on. When I get the boat raised off the trailer, I shore up under the forward portion of the hull, from the ground in such a way that I can maybe gain as much as I can in rolling the trailer forward. In other words, I jack the forward portion of the hull using the trailer to jack from, and let my jack down on the shoring blocks, and move the trailer, maybe a foot or so, and repeat the process. When the keel gets past the back of the trailer, I shore it up with blocks and lower the boat on them. Then I pull the trailer out from under the boat the rest of the way. It's a long slow process, but I've been doing it this way since I bought my new bunk trailer a few years ago. Prior to that, I had a roller trailer for this boat and never needed to use a hydraulic jack, but I did use the sailboat stands and was able to pull the trailer out from under the boat with my truck. Of course, the stern of the boat needed to be tied off to something solid, like a tree. Once you get the trailer out from under the boat, you can take two of the boat stands and move them forward about 6' or 8'. I would also shore up the bow of the boat with blocks, or a powerboat stand. It goes without saying that doing stuff like this is very dangerous, so you may want to think it out first and if you decide to do it, get someone to help you, preferably someone who is familiar with rigging. One thing to keep in mind is the boat's balance point when you shore up under the keel. You don't want the boat to tip forward. If you have plenty of blocks, you can shore up under the stern of the boat after you get the boat's stern raised. I criss-cross my blocks for good stability. I also have some blocks that I had cut length wise on a diagonal to make large wood wedges. These wedges come in handy for taking up a small gap.
Joe