Correct way to attach outlet wall plate?
There are a couple things you can do depending on what the cause is. 

If the entire cover plate stands proud of the wall, then it probably means the receptacle itself is not far enough back. Make sure it is screwed all the way into the box and that the box is set at a proper depth into the wall. If it's unfeasible to adjust the box, and you're only dealing with an eighth of an inch or so, build up the surface of the wall with some joint compound and blend it in as best you can.

If only part of the cover stands proud of the wall, it's probably because the wall is uneven. (Very common for plaster or drywall that's been repaired a lot.) Use some all-purpose kitchen/bathroom caulk around the cover plate where there are any gaps, and smooth it out with a wet paper towel.