I use eye droppers that I get the local drugstore. Use the TLAR method of mixing(That Looks About Right). I usually don't keep any on hand, just mix enough to use when I'm ready to weather and when finished dispose of the remainder.
I usually mix the thinner and paint in a bubble pack that over the counter cold pills come in. Put two full droppers of thinner in and then aid my color paint (black,sand, brown, grey etc). After I get a mixture that I feel will work, I apply it to one of my paint hulks, adjusting the mixture untill I get what I want (usually have to add color). Keep the mixture until I'm finished. If you are really carefull (don't spill it or put it somewhere that you can spill it) and cover it (piece of cardboard), it will remain useful for up to a week. When I'm finished, I just dispose of the remaining wash. Works for me.