The most important precaution, whenever possible use the original paint that was used on the wall to be certain that your colors match.
Try blending the paint with a rag technique. Dab a little paint onto an old rag and dab it off onto a scrap paper.
Use a sort of pouncing motion to add the paint to the wall. This method will add color in a thin and varied coat that mimics the original roller texture so that it's not a glaring new patch.
You may have to repeat several times to get the repair that you want.
If the paint still isn't blended in you may have to paint a block to blend the repair. A block would be an entire wall from break to break. In this case, it would be wall to wall. In another room, it may be from doorframe to window frame.
See, these techniques will leave you with a perfect paint job!