You can paint straight lines with masking tape, or you can paint straight lines freehand. Either way, if you don't have clean straight lines on trim or along ceilings, the job won't look professional. Luckily there are tips to make it much easier.
If you are painting both walls and trim, follow these steps. Paint the trim first. I know this runs counter to much of the advice you read, but this works. You can go back and touch it up later if you need to. Let it dry for a day. Carefully apply blue tape along all the edges. It looks best to have the tape overlap onto the trim by a tiny bit, like 1/16th of an inch. It doesn't look as good if the trim paint is on the walls. Cut in and paint the walls. Wait for the paint to dry for several hours before carefully removing it, otherwise it will pull off the paint.
For painting both walls and baseboards, the procedure is the same as in Step 1. Caulk and paint all the baseboards. It's even better to let it get on the walls. Let it dry for a day. Now carefully tape them off, cut in and roll the walls. Pull the tape off carefully when it's all dry.
Painting along the tops of walls can be done either with tape or freehand. It is easier to paint along the tops of walls freehand because you can use a larger brush than you can on trim. A 3-inch high quality brush is much easier to paint with than a smaller brush. It holds more paint so you can get a continuous straight line. Thin the paint just a little because it is easier to paint a straight line with thinner paint. Don't overthin it; just enough to make the paint go on smooth. Paint from your shoulders, not your elbow or wrist, in long, smooth bold strokes. If you get some on the ceiling, don't worry. Just touch up the ceiling after the wall paint is dry and redo the part you messed up later.
For painting along the top of a wall when the ceiling is textured or acoustic, use a sharp screwdriver or small putty knife to gently score a line along the juncture of wall and ceiling. Then cut in the walls as above. The lightly scored line will guide you in painting a straight line.
To paint a straight line along the ceiling with tape, use wide enough tape so you don't risk bumping the ceilings with your brush or roller when you paint the walls. Use a single run of tape from corner to corner. If you use short sections, the line will not be straight. If the paint on the ceiling is fresh, ask the paint store for "quick release" blue tape. Ceiling paint often doesn't adhere as well as wall paint and regular blue tape might pull it off the ceiling.
Here's a trick for painting straight lines on rough surfaces. Apply the blue tape as carefully as you can. Say you are painting a red accent wall and the adjoining walls are tan. You have the tape applied along the edges over the tan paint. Now, using a little tan paint, brush along the tape. Let it dry. This “seals” the inevitable bleed-through you'll get with the red paint. Then paint your red wall. Wait for all the paint to completely dry. When you pull the tape you should have a very nice line. If you're painting a rough wall up against something smooth and non-painted like wood trim, cabinets or panelling, use clear acrylic varnish as your first sealing coat.
You simply cannot paint straight lines without a high quality brush, so buy the best available. A good quality artist's brush is handy for touching up small areas where the paint bled through.