Block of wood under the bottom of the transom and a trolley jack.
The process of lifting it will usually see it centre itself with no other input from yourself.
If it doesn't, be prepared to lift it just off the rollers and try to physically shunt the back of the trailer sideways a couple of inches.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club