Plastic Pipets are the best/easiest/least messy way I have found to quickly mix paint.
I get packs of 100 from Carolina sciense supplies:
http://www.carolina.com/product/equipment+and+supplies/measuring+equipment/pipets/microchemistry+pipets%2C+micro-tip%2C+overall+length+5+3-4%22%2C+capacity+5.0+ml%2C+pk+100.do?sortby=ourPicks
Item numbers 736990 & 736988 are the most usefu.
Also pick up some "Mix & Save" containers (about $2.00 per pack) at you local Michaels craft store. They are small plastic cups with sealable lids that are great for mixing small amounts of paint and keeping them fresh for days.
http://www.craftsetc.com/store/item.aspx?dep=50&cat=2&subcat=57&IsOnSale=0&IsFeatured=0&IsNew=0&ItemId=79252
Just transfer some thinner to one of the cups with your pipet, then transfer some paint. To mix the paint and thinner--use the pipet. Draw the paint/thinner in and out of the pipet 4 or 5 times and that should mix it completely. Now just use the pipet one more time to transfer the paint to the airbrush. For acrylics, I usually keep a few baby jars near by (one filled with warm soapy water and one with rubbing alcohol) and use them to flush the paint out of the pipet as soon as I am doing mixing the pain. I can reuse the same pipet numerous times this way.