Leaving minecarts without moving them
Solution 1:
Place another minecart in a 1x1 hole within reach of your derailed minecart and just hop into it with a right-click.
(not my idea, shamelessly stolen from Minecraftaddict's YouTube, see his explanation)
Solution 2:
Since version 1.3.1, players are moved away from minecarts when exiting automatically, so as long as there is a valid block to stand on once you've exited the minecart should not move.