Here is the method that I use from http://www.his.com/~vann/KrgStuff/360turns.htm:
OK, let's say we want to back our boat (that backs to starboard due to propwalk) slowly into a narrow slipway. Start by lining up with your stern headed in toward the slipway and put your helm all the way over to starboard (and leave it there). (WHY put the helm to starboard? Because when you're backing slowly the helm has very little effect anyway -- especially with a full-keel boat, so putting it to port won't do much to counteract the natural tendency of the stern to move to starboard anyway, and we're going to need the helm to starboard for the "gooses" -- you knew that had to be coming, right?) Now, put her in reverse and give her enough throttle to get her moving in reverse. As soon as she's moving backward (and starting to turn to starboard), shift into neutral and coast. This will stop the propwalk and let you coast almost straight back. As you start to slow, shift back into forward and give her a quick goose. Because of the starboard helm, this will "hop" the stern to port without moving forward (completely offsetting the propwalk -- plus maybe a little more), and then you can then repeat the process. Now, this may sound a little complicated (you'll be following a slightly "scalloped" path), but you can back as slowly as the wind/current conditions will allow without terrorizing yourself or the other owners in the yard.
This method works great.
The best thing to do is practice and take your boat out.