You can put the boat on stands. Stands can be bought (about $70ea) or built from steel or wood. You jack the boat up, move the trailer forward until a x-member gets in the way. Set the boat back down (on the trailer and/or stands) and move the jack. Repeat until the trailer is slid all the way out.
Someone posted some pic from his trailer project on another site (see picture 3,4 & 5). http://www.picturetrail.com/sfx/album/view/16951430
Or, I'm sure any dealer can lift the boat off the trailer for you and put in on stands. I have no idea what they'd charge.