I keep a small rag with paint thinner on it to wipe my oil-paint tubes on after I use them. I already have it out and wet to start with. I do also have a collection of little black film carriers, so when the tubes split or get leaks or whatever, I can push the rest into these little containers, with a few drops on oil on top to keep them workable.
Tip from: Susan Tschantz
I use a toothpick or, if necessary the tip of a painting knife or sketching-lettering pen, to clean the inside of paint tube caps.
Tip from: Gerald Dextraze
I've seen it suggested you put a little petroleum jelly (Vaseline) on the screw thread when you first open a new tube. I haven't tried it, but I presume it stops paint drying in the thread. I guess moderation would be the key to ensure you didn't get any of it into the paint itself.
Tip from: Marion Boddy-Evans
If you can't get a cap back on, a bit of child's modeling putty over the tubes opening works well as it's reusable over and over again.
Tip from: Tina
If you're using oil paints, you don't have to put the cap back on straight away. So try this: when you are done painting for the day, light a candle. When there is a pool of melted wax, dip the end of the paint tube into it. This should form a seal which is easy to peel off when you are ready to use the paint again.
Tip from: John Najarian