Creativity needs freedom and space. Especially at the moments you are not deliberately trying to squeeze out a concept for an illustration, the best ideas seem to pop up. (in the shower, while taking a walk, in your dreams)
Also, to be creative, it is nessecary to leave in the possibility for your painting to fail. If you do not do this, and you are sure what is going to be the outcome, you will do the same thing over and over, and never learn something new...
In order to push myself to explore new techniques and concepts and to let creativity flow freely, I like to play this little game:
Before I get started, I make sure my equipment is ready. I set up a blank canvas and prepare my paints, or get my sketchbook and have my pencils ready. Then I set a time limit for myself. This can be 10 minutes, or a whole day. For this painting, I took 1,5 hours.
1: limited time: 1,5 hour.
2: In the first 15 minutes I come up with a concept, do little sketches, and look for reference images
3: In the remaining hour and 15 minutes I paint.
4: I try things I haven't done before
This way of working forces me to make descisions quickly. There is no time for hesitation, and often it results in strong choices. I learn ton's of new stuff doing it, even (or maybe especially) when it's not working out at all...