- Layers - Layers are your friend. Try to use as many layers as ossible. For example, if you are painting two colors, they should be on separate layers in almost all cases. Ditto for two textures.
- Layer Masks - Check out my tutorial on Layer Masks in GIMP. For me at least, it is night and day the flexibility you have when you start using layer masks vs without them.
- Paths - RobA's tutorial is a good start with paths for several features. What I have found, is that I tend to make paths for each channel as I use the Stroke Path feature quite a lot, and mostly I use it on the channels I create for my masks.
- Textures - go find some good sites. cgtextures.com is good.. but many don't tile without some change and you can only download 15MB a day without paying a subscription fee. One of the coolest I have seen is Genetica Viewer which has a collection of several hundred tilable images which you can then render and save are various resolutions. It is excellent for getting various rock structures, floor tiles, etc. I find that I render several copy of each base image at various resolutions and scales to my patterns folder so I have a variety.
Also, even though I have not used it much yet, you might want to look into getting Inkscape (free) as I believe you can export objects from there into GIMP as paths. RobA uses both, so he can verify.