Very simple hack I started doing years ago and realized no one else had, so thought I'd share.
A big issue I've always had when painting is that pouring paint into the roller tray it slops on the floor and also gets paint in the paint can grooves. Then when you try to put the lid back on it makes a mess and either glues the lid on, or the dried paint in the grooves makes it impossible to get the lid back on and then the paint in the can dries out.
To fix this:
Step 1: Cut the screw top from a shampoo bottle along with the pump.
Step 2: Cut a hole in an extra paint can lid large enough to fit the shampoo screw top.
Step 3: Put the screw top through the hold and attach the pump to the screw top.
Step 4: Put the modified lid on your paint can after you have mixed it.
Now you can just use the pump to get as much paint as needed and there is no dried paint to gunk up the can lid. Also, the paint in the can does not dry out, and you can accommodate both gallon and quart-sized paint cans by just trimming the pump stem as needed.
Hope that helps!