Cut in Quickly with a Steady Hand
Cutting in along trim that's not protected by masking tape takes a steady hand. And once you get the knack for it, you'll never want to fuss with taping trim again. Sure, cutting in can be hard for some DIYers, but you can learn to do it effectively.
Use a tapered paintbrush. The angled bristles uniformly unload the paint as you cut in. Dip the brush into the paint, then tap (don't wipe) each side against your container to knock off the excess. Brush the paint on the wall, about 1/2 in. from the trim. Then make a second pass, cutting in all the way to the trim. Avoid 'pushing' the paint with your bristles or you'll leave a ridge where you're cutting in. Apply just enough pressure to let the bristle ends glide next to the trim. To help keep the brush steady, move your entire arm as you paint instead of moving your arm only from the elbow down.
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.