Tips or resources for 'loosening up' my paintings

[Madzia]

In topic of resources Andrew Loomis wrote small chapter about edges and details in Creative illustration http://www.alexhays.com/loomis/Andrew%20...ration.pdf on page 101-111 (printed 101). I'm working on that topic myself and didn't see anything more comprehensive than Loomis.
For practice I found painting still life helpful for observing hard/soft edges.