I'm still cleaning up my brush presets- throwing out a lot of things I got from other people or found on the net. Probably 95% of those brushes aren't really useful.
Normally I use the one I already >blogged about< and it's still the most useful although it looks so plain. With nature it gets a bit more tricky. Of course it looks awesome to paint every single blade of grass but mostly you'll end up needing ages to do that.
Since I colorate my comic panels 100%ly and the backgrounds NEED a lot of work I redo my preset-list and try to create grass, flowers (those above are self-made as well) and mostly leaves for bushes or trees.
Since some people seem to be eager but have no clue how to do it... (and you SHOULD do it):
This video explains it pretty well
Keep watching it till the video end. The last part is the most important part of the whole process. Defining the brush presets will take a while and you have to test a lot but it's worth the work! :)
And because it's pretty interesting and I just got it: If you happen to have the awesome artbook of "Open Season" check the background paintings again. There's one page showing some custom brushes they actually used and it's interesting to see how those simple brushes work together and how amazing the final result looks.