I find it easiest to drive the sled right to where it starts to fall down the ramp, then with gravity as your friend and just pull from the bumber and ski tip and gently let it glide down while you pull a little. When the ski tips touch the ground, you can either continue pulling or just get up and ride it off. I also usually just drag the other sled over and repeat.
I have heard of people riding the sleds on from the front in an enclosed trailer, however I don't think I would try this on an open 2 place trailer, the first sled would probably work alright. However riding a second one on from the front doesn't seem like there would be room to do so, unless the ramp is really shallow.
If you're concerned about how close the trailer is to the truck, like stated above, partially "jackknife" the truck (however this could be a pain in cramped quarters.) You could also shop around for an extended, or longer hitch to allow more space between the truck and trailer.