One of the most frustrating thing when painting a model is to see paint wearing off as you handle the figure. There are of course various ways to avoid having to handle the model but sometimes it is unavoidable. One way to prevent this from happening is to varnish your model part way through the painting process, this will help to protect it during painting and provide a much stronger layer of varnish on the finished model. Just make sure that your varnish is of the same type as your paint, Acrylic varnish with Acrylic paint, and the next paint layer will go one without any problem.
This especially useful when painting larger models like tanks. I routinely apply a layer of varnish after the main hull is painted and then again after the stowage but before I apply weathering. That way I can wipe away unwanted wash layers without the risk of damaging the underlying paintwork or decals.