Ok, I'm not the best painter in the DIY world. I don't even enjoy painting that much. So I asked Alan Watkins from Sico paints to come in so I could grill him on my common painting errors.
I always thought that the thick rollers were for rough surfaces and the low mat rollers were for smoother surfaces. No says Alan. You need the thick rollers for latex so that it will carry enough paint to the job.
Another surprise for me, but this made sense when I thought about my past painting. Our first bit of painting doesn't seem to work nearly as well as after we have been at it a few minutes. That is because I never "conditioned" my rollers.
If you are working with latex, soak your rollers in water, then shake them out and let them sit a short while for the water to soak into the fibres. If you are using oil based paints, soak them first in paint thinner. You don't want the water or paint thinner dripping off of the roller, but you want all the fibres "wet". Then apply the paint. It will soak immediately into the roller, giving you more to work with on the wall.
Then I asked him about how my trim never seemed to match my rolled surface. There are two problems here. A roller puts on a lot of paint, but a cheap brush will not. Use brushes that have tapered tips that hold more paint and apply it thicker, rather than the square cut brushes that tend to scrape the paint off as much as put it on.
Then always paint wet on wet. I heard that before, but never applied it to the idea of the trim and the field. I always trimmed around the room, then came back to roll. But of course my trim was dry, and I got lap marks when I went over the edge of the trim paint with the roller. Alan recommends trimming two feet across the top of the room, then rolling two feet across by about three feet down. Then another panel two feet wide by another three feet down. Then trim the bottom and finish the field. Then go back to the top and do another two feet of trimming. This really helps to always be painting wet onto wet.
Get the paint on the wall fast with a loaded "W" stroke. Then spread it out sideways, then smooth it out with up and down strokes. That gets that 2 x 3 foot section done quickly, again, so that you are moving rapidly enough to always be working wet on wet.