er... :o That was something still in progress from a while back and I had never actually made the link 'public'. For the most part, I don't really paint like that anymore, and there are some things that aren't correct or could be better, like colour choice. I'll have to update when it's summer holidays again, and maybe move it to a place with more bandwidth
With your background - try to avoid filters, as they often appear very repetitive, and nature isn't like that. Also, the grass is too saturated green, and the rocks are unsaturated (unsaturated implies pure grey, saturated implies all colour. Objects in real life are rarely too saturated, and never ever unsaturated or pure black). It probably takes a fair bit of practice, but to make any kind of object (including rocks) not look flat you have to think 3-dimensionally. Most objects can be broken down into spherical/cylindrical/cube (hard edged) or a combination of those, so if you study the form of those, everything will become much easier. Define where the lightsource is coming from, then from there you can work out which sides of an object would be lit, which would be in shadow, etc.
I personally use a speckled brush nowadays for painting, as it's more like a brush and easier for blending, rather than the round brushes. If you want to use the grass brush, choose two colours as the brush uses both two colours that show on the palette (which is my guess as to why you have bits of orange in the grass
![Tongue :p](./images/smilies/tongue.gif)
). There should also be brush options in a tab to the top right (I'm assuming you use Photoshop(?)). You can make the brush jitter and all that, so that might be worth mucking around a little with for making painting easier.
I think I've rambled on a bit too much... but this site has really good stuff on colour theory:
http://www.itchstudios.com/psg/art_tut.htm