Leveling--what is it?
Ever wonder why some paints look much smoother than other paints when they dry? It is called leveling.
There are many factors to getting a level surface. But first and foremost you need a paint the levels well. There are many paints that have been designed to do this specifically such as Benjamin Moore's Advance and California;'s Nextech and Ultraplate. These paints have been designed to give an ultra smooth finish on trim and cabinets. However, it is not as simple as putting on a coat of one of these products to get the smooth finish.
The best way to get a smooth finish is to spray the paint. However, most non profession (and many professionals including myself) do not own a sprayer. But, if you willing to put in the work, you can get a finish that shows almost no brush marks at all.
In order to get the almost ultra smooth finish without using a sprayer you will have to put some work in. First, if you much clean the area to be painted thoroughly. Next sand, sand, sand. If has been previously painted I suggest starting with a rough sand paper such as an 80 grit and then moving to a 220 grit. If it has not been painted previously you will have to use at least one coat of primer and then sand with a fine sand paper before putting on the finish coat. Vacuum well and then right before you paint use a tack cloth to remove any left over dust. The pint of all of this prep work is that if the underlying coasts are not smooth or there is dust is will keep the finish coat from being as smooth as possible. Also the dust will keep the finish coat from adhering.