I can never seem to paint a wall from start to finish. I get bored or get pulled away to deal with the girls/work/house.....whatever seems in that moment more entertaining than painting. At the same time I hate wasting paint. The easiest and cheapest way to keep paint in the tray and usable until you are ready to get back to work is with plastic wrap.
Once you are done for the day simply cover the pan, roller and any other painting supplies with plastic wrap and make sure it goes around the sides. This picture is from paint that has been sitting for a day and it's still wet as you can see. I've forgot about paint for 2+ days and come back to it and it's just like it came out of the can.
Halfway done with priming the basement. Let's see how many days it takes me to finish. :)