Despite how long I've been painting, I still make a lot of rookie mistakes. Most of that is due to being lazy.
1. Shake the paint. Before you even open the paint, you need to shake it. Shake it for a good minute or two if you can. The elements of the paint can seperate over time. This is true of washes as well. Hell, might be more true of washes.
2. Thin the Paint. Paint is generally too thick. For me, for the most part, it's not a problem. The details don't really get covered but the coverage itself isn't necessarily as smooth or taking to the layers like it could. I keep hearing the term 'milk' used as to how thick you want the paint to be.
3. Don 't overload the brush. You just want something on the tip of the brush. You dont' want the paint getting up into the metal part of the brush past the bristles because when that stuff dries, it spreads open the brush and then you're awesome brushes have become the crap.
4. Clean the brushes. There are a lot of good cleaners out there. See some reviews. Do some experiementing. Find the one that works for you.
5. Keep the Brushes Upright. Don't leave them laying down. Don't leave them laying in the water. Don't leave them with paint on.
6. Don't mix paint with the brushes. Use a toothpick or an old brush if you're going to mix paint.
I'm sure there are a ton of things I'm missing but those are some of the sins that even now, as I'm telling you to do as I say, not as I do, I still make. Everything counts in small amounts eh?