This tends to happen to me as well, but the way I get around it is I have 3 models on the go, diff types, I go from one to the other some times makes a nice change.
Im not really doing army work at the moment so I tend to test diff colour schemes out, specially with my new Tau purchases. Painting a large army of the same types I can see becoming tedious very fast. If you are like you say painting an army, rotate between minis for example, paint a space marine then onto a Vehicle then a Terminator and vary your work. Also try and make each model different slightly, via markings, small changes in colours, but still go with the overall theme.
Variety is the spice of life!
Good luck with it!
