You probably already know the following things, which I have slowly picked up over the years, but just in case you missed some, here they are.
When painting, your results will reflect the opposite of the lighting conditions in which you are working. If you are in a bright space or outdoors without shade or an umbrella, your painting will be dark. If you paint indoors under cool artificial light, your painting will look warm in more normal situations. And so on. I believe that generally Old Masters had dimmer studios, perhaps with a small north light window. Then they got light and warmth 'into' their paintings. Our eyes are so amazing that they just adapt to all conditions when we work, and we may not consider where we are painting. Heck, painting is hard enough all by itself.
By the way, speaking of Old Masters, how come so few of us contemporary artists (meaning alive - a pet peeve) do representational work that is as good as the work done before there were cameras or even electricity? You got it - they worked from life.*
Because...
It's not about a particular piece of art - it's about the process. As I work, it's not unusual go from thinking that a particular piece is a masterpiece, to thinking it is a disaster. And this seesaw can continue even after it's done! Have you had the feeling of seeing an older piece and thinking "it's not half bad"? Or even "I used to be better than I am now"! That hurts! Well after painting a lot for twenty plus years I believe that it's about the process. Just do the work and let the evaluation take place whenever. Surely we all want to do great work so just lighten up. Just make sure the process is solid.
On a more practical level:
Egberts. Try 'em. They are all I use. They vary widely from soft (Rosemary) to stiff (Silver) to in-between (Robert Simmons and Richeson and many others). Egberts seem to be the number one thing folks take away from my workshops.
And by the way:
Stop cleaning your brushes! My friend Bart Lindstrom was painting in my studio, and at the end of the day I asked if he was going to clean his brushes. He said he just wipes them off after swishing in mineral spirits. What??? All the hours, the soap, the water...
Speaking of brushes, try using a larger brush than you are quite comfortable with.
Speaking of turp, I seem to be using far fewer brushes and a lot less turp and paper towels. The idea is to go from mixture to mixture. Sure, now and then you need to really get a clean brush, and mix a pristine mixture, but more often than not, you will get 'unity' by being less neurotic about paint, palette, turp, etc.
Speaking of unity, that's actually the entire deal with all creative expression. We get so obsessed with craft and all that goes with it (which is good) that we forget what inspired us to be artists in the first place (not so good).
But wait, there's more:
Freeze your paint. I put my palette in a box in the chest freezer. My wife is not enthralled, but I hardly throw out any paint at all.
Wipe paint off of yourself with oil, not mineral spirits.
Stand on a pad. Save your back! Standing is good because your point of view changes more. Keep your painting vertical and at eye level.
Take breaks! It is unfortunate that as we progress deeper into our work, we can get fatigued and not make good decisions. I always say a painting is just a million decisions. Always OBSERVE, MIX, AND LAY IT DOWN. I define fatigue as thinking that whatever mixture happens to be on your brush is the right one for everything all of a sudden.
Paint a good bit in standard sizes. I used to stretch all kinds of weird sizes. Now I do 11x14, 16x20, 20x24, 24x30, 30x40... you get the idea. I still do non-standard sizes when necessary, and sometimes in the field I'll do something like paint two 10x24s on a 20x24 to get long narrow compositions. Then I have to cut them down, mount them, and order custom frames. That's OK, but it's nice to have a large supply of frames to interchange and help get things out the door and on the wall.
Loose painters want to be tighter, and tight painters want to be looser.
Be grateful for every day you get to make art.
I think that's it.
*Hey, I'm as guilty as anyone of using the camera for portraits, but I learned working from life, and always try to have at least a weekly session to continue to work from life. All of my still life and landscape is from life, and I aspire do all work from life before I pass on.