This is a small tip for those kinds of situations. I heard about the idea from another artist, who uses it all the time.
All you need is a small painting tray. I bought this one at a local hardware store. The tray was made for small detail paint rollers, and was inexpensive.

I filled the deep end of the tray with a moderate amount of linseed oil. Brushes are laid on their side so that the bristles rest submerged in the oil.
The oil retards the drying time of used brushes, so dirty brushes can be stored for relatively long periods. Later, if I need to use one of the brushes in the tray, I take it out, and wipe it clean on a towel or rag. The brush can be put to immediate use, or given a real cleaning.

