Cover Paint to Keep it Fresh
Paint dries fast, even paint in your roller tray. If you need to take a break for more than 10 minutes, cover the paint. Place a lid on your 5-gallon bucket of paint and a damp cloth over your handheld paint container. Use aluminum foil to cover your roller tray. If you don't, the film that forms on top of the paint may end up on your walls. Here's a trick for storing paint brushes overnight.
Use Extenders to Avoid Do-Overs
The longer that paint stays wet on walls or woodwork, the fewer lap marks and runs you'll have to deal with. Lap marks are those dark, ugly lines caused by painting over an area that's already dry. If you can keep the paint wet longer, you won't have to worry about them. And the way to prolong the 'open time' of paint is to add a paint extender or a conditioner (Floetrol is one brand; at home centers and paint stores).
Pour all the paint you need for the room into a 5-gallon bucket, then stir in the extender or conditioner (following the manufacturer's recommendations). Because extenders and conditioners help level out brush marks and paint runs, you won't have to go back and fix them later.
Cut in One Wall at a Time
Once you have your paintbrush in hand, it's tempting to cut in along all the trim, the ceiling and the corners in the room. But you'll get better results if you cut in just one wall, then immediately roll out the wall before cutting in the next one. That's because if you roll out the wall right away, while the cut-in paint is still wet, the cut-in paint and the wall paint will blend much better, reducing the chance of lap marks.