Find something different to paint.
That's right, paint something you don't normally paint. Something completely different than what you're used to painting. The further away, the better I think. If all you ever painted were Space Marines, try something from the Fantasy range or better yet, a different game all together like Warmachine, or perhaps a Talisman minature!
You'll be surprised at how difficult it can be. Even if you use the same colors you normally do. Just the different model, textures, scale and such will throw your game off. You'll find yourself focusing on things you normally take for granted. You'll hit bumps in the road where it's normally smooth sailing.
All of these things will help you get better. You might find a new technique or trick you never knew before because you've had to improvise this time around. You might find a new color combination or method for applying paint you never tried before. All things that will improve your work when you head back to the normal stuff.
While it may not be as big of a leap, it will still help you out with your painting overall. Never done a quartered Space Marine scheme in contrasting colors? Give it a go on a test model. You can always strip the paint and never tell anyone you painted another chapter. Your friends don't have to know.
And it doesn't have to be Golden Daemon stuff either. In fact, you don't even need to finish the model if you don't want to. Sometimes I'll paint a shoulderpad just to try something out. The point is, you need to get out of your comfort zone when it comes to painting and try something you haven't done before.
