No, it's not paint a zombie, but it's close. Actually I'm willing to bet there are any number of things you could do to improve your painting, but this would be my top recommendation.
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.
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.
You have no idea how hard this model was to paint
I've painted a few models, meaning I can count them all on one hand, that were "different" from what I normally do. And I learned something from every one of them. If you don't have the ability to go out and buy something different to paint, you can try pushing yourself to try something you haven't done before.
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.