How to prevent acrylic paint from drying?

I think everyone has her own good way to do this. I use small tubes bought from art shop and a spray. I squeeze the color into the tube and constantly spray the water into it to prevent it drying during a painting session. Don't spray too much water into the tube though. Otherwise, the color becomes watery.

After finishing a painting, I cover the tube. The tube has a very tight lid. The color in it won't dry for months.