Make sure when you paint you do a wall all at once, and make sure you leave a wet edge, meaning don't let an edge dry and then try to start at the dry edge, you will get roller line mark.
Its best to do a small section (4' wide) from ceiling to floor, in that section i do the top and bottom seperately making "W"s to coat the wall. Then after it is painted, then take the roller (without adding paint) and move it from the ceiling to the floor, one roller width at a time over the area you just painted, to remove any beads and smooth out the finish. Next then start the next section from the wet edge you left. Only stop when you are done or you get to a edge in the wall.
And DONT paint over dry paint unless you are putting on a completely new coat.
"If the world didn't suck, we'd all fly off."