Like Tim, I pre-mix
Tamiya and Mr Hobby Lacquers in the jar and put either a "T" on the lid to show it's been thinned or "TR" if it's thinned with retarder-type thinners. I tend to do that with a little of the paint mix, which also instantly shows up which jars have been opened and exactly what the colour looks like in reality. As soon as I've taken paint out of the jar, the cap goes straight back on to minimise oxidation. Generally, I tend to use the paint fairly quickly so I've not encountered it going off - had some around for a few months and it's still OK. The experts on this site will be able to let you know how long you can get away with.
Did try a pre-mix with Humbrol enamels once but they didn't even last a week.
Personally, I would be a bit nervous of using any squeezy bottles for paint - once the squeeze is finished and pressure on the bottle is released, air will be sucked back into the bottle through your paint mix. I would have thought that action would accelerate the paint going off.
I definitely wouldn't put thinned paint back into the original paint. Only thin as much paint as you're going to need. Otherwise, use a good quality paint and go for the approach above.
Then, as Tim says "Any unused paint just goes back in the bottle." (of thinned paint, that is)
