If you really want to loosen up, try painting faster. The problem with most drawing and painting is that the dang brain gets in the way. During a deep, art conversation a friend of mine whom happens to be a drummer, recommended a book to me. "How to Golf" by Jack Nicholas! Weird choice, but hey he's a musician...who can figure those guys out? I found this 11-page book in a used bookstore, for a mere loonie. In a nutshell here is what Jack had to say about golf...
"Monday thru Friday I practice golf. On Saturday I play golf. When I play golf I never think about it. I just play. Monday I begin to practice all that went wrong with Saturday."
It went on to say when its time to play, the best thing you can do is just live in the moment. In comparison, swap those golf clubs for brushes and just paint. Don't think. Whatever you don't know will be obvious and that is just the thing that's crying for correction.
No thinking while painting! The old gray matter will usually make the best decision right out of the gate. The, ‘ooh, that was my first thought', thought is usually correct! When you finish a painting session, this is the time to step back and let the left-brain analyze, not during the act of painting. It can be hard to keep the brain's influence out of the decision making when painting, but painting fast is very effective for controlling this. The brain not only likes rapid-fire decisions, it is very good at it.
When you paint rapidly you don't have time to second-guess things. Try a sketch or rapid drawing in under 5 minutes. Perhaps painting a 9x12 in less than 15 minutes. Work the biggest shapes first, and I mean really big. There should be no more than 5 shapes to start. Pare these shapes down into progressively smaller shapes. You can't paint the wormhole in the apple until you have painted the apple first. Declutter at the beginning. Take out all the detail. You can put in final details right at the end. Remember these are not meant for public consumption. These are studies, so feel free to play your heart out!
Painting faster will loosen you up as well. It frees you up to not second guess yourself allowing bigger bolder decisions, not getting bogged down in that crummy detail. Some of my best ideas came from paintings that were done in a rapid manner. Plus it's just plain fun.
Try a few studies in rapid succession. Kinda like jamming in a band. You will be amazed at how loose you can get and still retain your original idea. Working in a fast paced manner will allow you to see what you truly know. That drummer friend of mine always told his students,