I try to save up until I have enough parts that need to be painted a similiar color, then paint them all during the same session, not only for ease of cleanup but also to be more efficient in using paint. Of course, sometimes a model ends up on hold because I'm not willing to bust out the paint for just one small piece.
I usually don't work on more than two models at a time nowadays. Like I-beam, I used to try and build multiple models at once (5-7) and had an assembly-line mentality. But i think the quality of the models suffered so now I am more patient.