Enhancements can come in a variety of ways, all of which result in differences between the left and right channel that give a sense of space that wasn't there before.
1. EQ each channel separately. This is a very effective way to move a mono sound source out of the center without affecting phase (mono compatability).
2. Delay one side (or one set of frequencies on that side... often more effective because you can avoid the bass frequencies). This is known as the haas effect and *will* mess with phase between the two sides, as that is how the effect is achieved. But used subtly and in combination with the above EQ trick, you can get away with some great results with little damage to the mono signal. Be sure to check the results in mono, I often make the changes while in mono until the mono sounds pretty good bypassed and unbypassed, then check it in stereo again.
3. Comb filtering one side. Again causes nasty phase issues but can be effective if used gently. Try a plugin called Stereo Pan by Jan Krueger, in subtle mode.
4. Pitch modulation on only one channel. With a slow rate and shallow depth, a chorus effect 100% wet applied to just one channel is another effective way to create stereo difference.
5. Mono > stereo reverbs, and stereo delays. This is the most coloured sounding method as you are now changing the room sound of the source, but a little bit can go a long way to 'popping' a mono channel out of a stereo mix. I often use stereo delay on vocals for this reason.
All the above used in combination can be a safe bet but also get messy. Save some track templates when you find something you like, it's really worth it. I have a mono > stereo track template that uses the Stereo Pan plugin and some left/right EQ to achieve a pretty good stereoization without ruining the mono signal. You can hear the results here, and if you'd like I'll post the template for you.
https://www.youtube.com/watch?v=iWT4ssB4iUY
In the video each example audio is sequenced as so:
Mono, stereoized, mono again, mono mix-down of stereoized version.
This way you can hear all the iterations and how they compare. My favourite result was on the keys.