Woodwork and wood surfaces.
Helen: I use a good quality paint (that really does help.) Or... you can use a foam brush or even spray paint
The key to a good smooth paint finish is to use a good quality paint and definitely a good quality paintbrush! The finer your bristles the smoother your finish. The folks in the paint dept. can advise you which brush is best for the paint you've chosen. Good Luck!
I painted my oak cabinets white, and I found that foam brushes are good for avoiding brush marks: http://momhomeguide.com/my-painted-kitchen-cabinets-a-year-later/
Before you begin, use proper prep. Make sure the woodwork is very clean and deglossed with a light sanding or wiped with a deglosser, available at any home store. If it's a previously painted surface with existing brush marks or chips, sand those out as best you can, or they'll affect your final results. If this is unpainted, stained or varnished wood, you definitely need to sand and prime first.
Good quality paint and paintbrushes, as suggested, are a must. Look for a paint that says it's self-leveling. Personally, I am not a fan of the so-called all in one paints, with the primer in the paint. I find that its consistency drags and makes for more brush marks. I much prefer an initial coat of primer, with a light sanding after it dries, then the paint. There's also an additive called Floetrol (for latex and acrylic paints)or Penetrol for oil-based paints,) that helps with brush marks. It does not change the color or coverage. It gives the paint more "open time," that is, keeps the consistency wet a bit longer, so those brush marks don't set in. They kind of gently melt together. It takes practice to get just the right amount of additive for your particular paint, but once you get it, it's like magic..
Technique matters. Try not to work the paint back and forth too much; that creates brush marks and defeats the self-leveling aspect of a good paint. There are lots of great video tutorials on technique online that you can search.