Here is a method that was given to me on another forum in my P+M blog.
#1 Thing to ask about: Do you use a palette?
If yes, is it a wet palette?
If no to either of these, make a wet palette! What you'll need (and I'm typing this up for more my benefit to copy/paste later on so if you have a wet palette already, feel free to skip down):
1. A piece of tupperware, with a lid, that you don't want to use for food anymore. Clear with parents/Significant Other before commandeering one, of course.
2. Paper towels.
3. Parchment paper (wax paper will not do).
Steps:
1. Put the paper towels in the bottom of your tupperware container. Cut to size if need be. You'll want a decent thickness.
2. Add water do the paper towels until they're soaked through. You've just made a sponge of sorts. Make sure there's enough water to where the paper towels are saturated.
3. Cut some parchment paper to fit on top of the paper towels. Get four paint pots and put one at each corner to keep the paper from curling up. After a while the paper will be soaked through with water and will no longer curl. Return your paint pots to their home.
4. Ta-da! You now have one of the best tools in hobbying! Why? Let me count the ways...
Why this is awesome:
1. The biggie is that you can get more working time out of your paints. Scoop some out of a pot with your brush, transfer to the palette, and voila, that paint will be good for days. You just have to close the lid nice and tight when you're done painting (or get up to have dinner or whatever). Be sure to keep those paper towels soaked.
2. Mixing paints is more reliable. By not having to remix your paints more often (which happens a lot when highlighting and you don't want to spend tons of money on new paint pots just to use as a highlight), you have more consistent colors. Yes good.
3. I can't emphasize how awesome it is to keep paints good for days at a time.
Edit. Probably the same method shown in your link