Pick a theme (or two, or three) and work on it. And keep a notebook! For me, these are usually catchall books containing model ideas, sketches, partial diagrams, partial CPs, tree figures, etc, etc. A filled notebook is a wonderful reference (plus it makes you laugh at yourself, when you see the horrible things you did previously).
My own design process usually goes like this:
1. Decide what I want to do.
2. Google for pictures and descriptions.
3. Draw a rough sketch showing the pose and profile, taking note of the key features.
4. Convert sketch into tree figure.
5. Convert tree figure into circle/square packed CP.
6. Do some paper wranglin' to get the durned thing to collapse into a workable base.
7. Fill in details, details, details and details.
8. Repeat steps 2-7 until I'm satisfied.
I find it's usually good to work on 3-4 models in parallel, so if you get stuck on one, you just move to the next.As for themes, well, it seems to depend a lot on what your other hobbies are. The main ones I'm working on at the moment:
- d20 critters
- British mythology
- Chinese mythology
- Battletech vehicles