The real challenges faced when painting baseboards are: keeping paint off the floor and or carpet, keeping dust (from the floor) out of the paint, and getting a clean line on the top of the baseboard where it meets the wall paint.
Lets look at each of these items.
Keeping paint off the carpet
The keys to painting baseboard at the carpet line is to paint below the carpet line. You do this by masking off the carpet, and pressing down on the carpet with a paint shield as you paint.
Let the paint dry before you pull up the masking tape or the carpet will “spring” back up into the wet paint. When removing the tape (once the paint is dry), first put a spackle knife down between the baseboard and the taped carpet to break any bonding that may have occurred between the two upon drying.
Keeping paint off a hard floor
This is pretty easy, if you are not steady with brush painting, tape the floor with masking tape (use blue tape if you are leaving the tape down for more than a few hours). If you are good with a brush you don’t need to do this, just cut in the bottom of the baseboard first (a few feet at a time), then cut in the top at the wall, merging the two swaths of paint in the middle.
Keeping the paint clean
This is important. You MUST vacuum the floor perimeter at the baseboard / floor to pickup all debris. This is true for wood floors, vinyl floors, carpeted floors and especially true for new construction where there will be a raw plywood floor. If you touch the floor with your brush, you will pick up all sorts of dirt and fibers that will be transferred to your paint can upon re-dipping, and to the baseboard itself directly. This will result in a dirty sandpaper like finish on the baseboard. Clean the floor first.
Clean lines on the top of the baseboard
I prefer not to tape the wall, but I am a professional so cutting in straight lines is a daily practice. If you do not do this everyday, you might need to tape the wall to get a good line. The 3M 2080 blue tape will do a great job but it is expensive, another tape that works well is the often overlooked brown paper tape.
Painting against tape.
You will quickly learn that if you don’t use a few pointers that masking tape is almost useless in getting a clean line up against a roller textured wall. The paint is prone to bleeding under the tape, leaving lots of touch up. Because of the hassle involved with masking clean lines, painters usually learn early on to avoid the whole mess and learn to paint straight lines - free handed. See How to Paint Straight Lines
If you do mask, burnish down the tape edge where it meets the baseboard. Use a piece of cardboard as a burnishing tool. You must use a delicate surface, easy release tape for this masking (brown paper, or 3M 2080 tape), or you risk pulling the paint off the wall when removing the tape.
Use a dry brush when brushing against the tape. A heavily loaded brush with find a way to ooze beneath the tape - somehow. Try not to work the paint under the tape. Keep your brush strokes few and simple up against the tape.
Finally, remove the tape from the wall BEFORE the paint dries or you will have a real mess. I know this is the opposite of when you remove the tape on the carpet, but they are different situations. Remove the tape on the wall BEFORE the paint dries, remove the tape off the carpet AFTER the paint dries.
Despite your best efforts, you may still get a few spots of paint bleed. Just go back and with a steady hand and small brush clean up the line with some wall paint.