We have concrete ramps so power loading is not an issue. Wife reverses trailer into water until 2-3 feet of bunk sticks out, then simply drive the boat on. Usually trim up about half way. Wife attaches winch for the last foot usually, although sometimes I can drive it easily all the way to the roller.
Yep, a crosswind is definately a problem! In these conditions I maybe don't have the trailer quite as deep (maybe 3-4 foot of bunk out), and I find you have to keep a nice steady pace (about a brisk walking pace) to maintain boat control and compensate for the wind blowing you off course. Then I approach nice and steady at brisk walking pace from say 40 foot out and drive it on the trailer in pretty much one shot. The boat will centre itself unless you are a fair way out.
Not a perfect science, and takes some balls the first time, but it works for me...

If you can't power on, then you need to run a rope from the stern on the upwind side and have someone hang onto it (from either upwind on the wharf or shore) while you winch on. It's this rear line that will keep your boat straight