There are four things you can do to help minimize brush
marks and leave a smooth surface on your paint job:
Set your loaded brush down on a dry section of the wall -
or the trim, or siding, or whatever you're painting - and
to lift it from a previously coated, still wet section.
Make sure your paint is the right consistency. Thicker
paint holds marks more than thinner paint does. If you've
left paint in a closed can overnight, it will dry out and
thicken up a little. Thoroughly stir the paint and, if it
seems too thick, add thinner sparingly - you don't want to
get it too thin. Stir it thoroughly and try it again.
The right brush and to make sure it is in the best
condition. Oil-based paint, shellacs and varnishes should
be applied with a china bristle brush.
Latex paint and stain should be applied with a good
quality nylon/polyester brush. These are often labeled
"for use with all paints." Better synthetic filament
brushes have more 'flags' - the splits at the end of each
filament - and the filaments are round and solid, not flat
or hollow. In other words, they more closely mimic real
bristles. Good quality nylon/polyester brushes are only
slightly less expensive than good china bristle brushes -
say $30 for a 4" sash brush, for example.
There's also the issue of the cleanness of the brush. If a
brush has not been thoroughly cleaned, all the way back
into the ferrule, and the bristles and filaments combed
straight and wrapped in a cover to dry, they may start to
bond together into small spikes.
There are four things you can do to help minimize brush
marks and leave a smooth surface on your paint job: 
Set your loaded brush down on a dry section of the wall -
or the trim, or siding, or whatever you're painting - and
to lift it from a previously coated, still wet section.
Make sure your paint is the right consistency. Thicker
paint holds marks more than thinner paint does. If you've
left paint in a closed can overnight, it will dry out and
thicken up a little. Thoroughly stir the paint and, if it
seems too thick, add thinner sparingly - you don't want to
get it too thin. Stir it thoroughly and try it again. 
The right brush and to make sure it is in the best
condition. Oil-based paint, shellacs and varnishes should
be applied with a china bristle brush. 
Latex paint and stain should be applied with a good
quality nylon/polyester brush. These are often labeled
"for use with all paints." Better synthetic filament
brushes have more 'flags' - the splits at the end of each
filament - and the filaments are round and solid, not flat
or hollow. In other words, they more closely mimic real
bristles. Good quality nylon/polyester brushes are only
slightly less expensive than good china bristle brushes -
say $30 for a 4" sash brush, for example. 
