It's tough to completely screw up painting a room—dump the paint on the floor maybe?—but painting a room really well isn't that easy, either. Home blog Re-Nest's guide to painting a room efficiently offers a few expert tips.
Re-Nest's guide to room-painting guide covers everything from making sure to remember to remove all the fixture and switch covers before getting into painting to the importance of edging in the areas you're going to paint for nice even colour and clean edges. If you've helped paint a room before but never done the cutting-in part, it's worth a read:
Cutting In: This is the most time consuming, but most important step to ensure a good looking paint job. When using the brush we just dip it directly into the paint can rather than pouring into a separate container to minimize wasted paint, but never dip a brush more than 1/3 then length of the brush or else you'll risk ruining the brush because it will be very difficult to clean. When handling the brush, hold it as you would a pencil for maximum control. Press the brush lightly against the surface, then, as you move the brush, add just enough pressure to make the bristles fan away from the direction of your brushstroke. The bent bristles and the pressure will release a fine bead of paint that will spread perfectly along the edge you are creating.
Beginning at the corner of the room, use your angled brush to cut in (also known as edging), applying a thick band (2-3" wide) of paint along the perimeter where you're cutting in. Do this in 4-5" long sections to ensure precision and a sufficient coating of paint. You will need to cut-in around all trim, ceiling/wall intersections, inside corners, and anywhere there is a change in colour.
Check out the link below for the full guide, and if you consider yourself a bit of an expert hand at room painting, share your expertise in the comments.
How-To: Paint a Room [Re-Nest]