My machine has a walking foot plate with a little flange in it. The flange rides in the ditch and you get an almost perfect result. I have a Viking, but I know that this walking foot is available for other brands.
http://www.husqvarnaviking.com/en-US...r-s-Guide-Foot
You really want to use a walking foot, but you can also try an edge joining foot. Same type of deal, flange in the center of the foot. These should be available for any brand machine.
http://www.husqvarnaviking.com/en-US...e-Joining-Foot