Right now I'm following the sugggestion on the Minecraft wiki. Stone block with switch on top/powered track/regular track/4 powered tracks. When the cart comes into a stop, it hits those 4 powered ones and gets a hard boost into the switch block. If I'm not fast enough and miss hitting the off switch, the cart will slam into the switch block, then rebound backwards onto the 4 powered blocks and off I go again.
I wonder if putting the 4 powered ones further down the track would do the trick.