There are basically 3 safe ways to lift your boat from the trailer for bottom service:
1) If your boat has the optional lifting eyes you can attach with whatever you have available to lift from these, overhead crane, chain fall from a suitable beam, even forklifts with a chain or strap. Watch the eyes carefully when lifting, as boats get older these eyes may need adjusting, watch the eye if it pulls away from the deck as you are lifting the eye needs to be tightened up.
2) The most common way is to use the transom eyes & bow eye. These are designed to handle the load. If a boat is in my shop & has both lifting eyes & stern & bow eyes I prefer to use the stern & bow eyes for a couple reasons, first the lifting eyes may cause cracking in the gelcoat on older boats especially if they are seldom used, second, I like the stability of a 3-point lift vs 2 point where the boat is able to swing. I use (4) 2,000lb. cap. straps with flat hooks on the ends, 2 in the back, 2 in the front using a "spreader bar" that is 2' wide that takes the stress off the bow. NEVER WORK ON A BOAT SUSPENDED, ALWAYS SHORE WITH WOOD BLOCKS OR PROPER STANDS.
3) The Cajun way, (never try this on an inclined driveway or on dirt/gravel) As described elsewhere on the forum this method works on a budget. Run the trailer jack all the way down, block the back corners of the hull, then lift the bow up, (18s, 19' advances the jack will work, bigger boats use a floor jack, the dolly jack can't handle heavier weights) When the front goes up the the back of the trailer will go down placing the weight on the blocked corners. When all the way up, shore under the keel & let the jack down & pull the trailer out a couple feet. Here is where it gets tricky, to completely remove the trailer you have deal with the crossmembers in the trailer. This can be accomplished with a floor jack & another set of shoring blocks alternating between spaces moving the trailer out a bit at a time. A better way would be to rig an adequate overhead chainfall or hoist for the front only.