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• I use small brushes for detail (either round or pointed round), and (smallish) flat brushes for slightly larger areas of color. This is probably the most important factor. The small brushes allow more control over the acrylics. You can read more about the different types of paintbrushes for acrylics, and what they can do, on this page about artist paint brushes.
• When I am painting straight lines or need straight edges, I use artist's tape. (Artist tape is preferable over "regular" tape because it is easily removable and won't leave behind any visible residue.) I apply the tape gently but firmly where I want to paint a straight line, and then apply the paint right next to it, with part of the paintbrush (and the paint) going over the edge of the tape. That ensures that the line goes right up to the edge in a nice, even manner. I then wait for the paint to dry before removing the tape. It's important not to load your paint brush too heavily with paint when you use this technique, because if you do, after it dries and you remove the tape, there might be a jagged ridge of paint all along your straight edge. In some cases, you may not mind that at all, but in other cases it may not be desirable - so it's something to be aware of.
• I paint with high quality acrylics, which allow pretty good coverage. Sometimes I can get the color I want with one layer, and sometimes it takes two layers, but usually that will be enough to get the color I want. Artist-quality acrylics have more pigment than student-quality acrylics, so they give better coverage. Student-quality acrylics have more binder and filler, so it can take more layers to get the color or coverage that you want, which provides more opportunities for mistakes.