It’s better to plan on applying 2-3 thin coats of paint (over primer), than trying to cover with one. Opacity will build up with subsequent coats; don’t worry about opacity on the first coat. This is particularly true with light colors.
When covering larger areas (decks, hull, etc) use a larger brush. Tiny brushes hold very little paint, and as the paint dries in the brush your strokes will become streakier. A larger brush has a larger ‘reservoir’, and paint flows more readily from its fibres. Make long, even strokes when you can and DON’T let puddles of paint build up; brush them out.
When covering larger areas work quickly, in one direction if possible- you want to avoid brushing back over drying paint after the first few seconds. It will get sticky and make streaks.
If the paint is too thick (i.e., it doesn’t flow readily from the brush, it piles up, etc) thin with just a little mineral spirits. Remember, you can always do another coat for opacity. One thick, sloppy, puddly coat of paint will obscure more detail than four+ thin coats.
Let each coat dry COMPLETELY, i.e., overnight or longer, before overcoating. The previous painted surface will remain somewhat vulnerable to the solvent in the paint for hours after it ‘seems’ dry. Be patient.
When everything is finished, apply a sprayed clear coat. By creating an even surface it will greatly de-emphasize brushstrokes.
Throw out those ragged old brushes. Buy new ones. A new cheap brush with a point is better than an old, worn-out, shedding expensive sable one.
Never ever remove excess paint on your brush at the rim of the tin: It’ll dry there and crumble into your fresh paint when you close the lid.
Dip you brush into appropriate thinner before taking up paint for the frist time. It’ll ease the flow and will make cleaning the brush a lot easier.