How to keep my acrylic brush from hardening?

I am new to doing acrylic nails. I want to go to college to be a nail tech. When I did them for the first time (which im sure I didnt do them completely right) but my acrylic brush kept hardening as i was applying it to my nails. I know it said clean it between applications, but what do i clean it with? I thought if i put it back in the liquid acrylic it will stay soft. Someone please explain to me the process. Any nail techs?