Here is a tip I thought up the other day when I found some foam blocks for another project, I cut a notch out of the top and use them to hold models for airbrushing. It gives access to three sides of the model and you can have two at once or with a larger square design you could have four. While you are waiting for the paint to dry you just pick up the foam and start painting another set of models. The foam also catches alot of the overspray, creating less mess and fumes.
The other is a milk frother for stirring paint, I got mine at a kitchen shop but have since found cheaper ones at Ikea for $2. Mine cost about $7 and fits into most model paint jars with no problems, don't worry about the froth bit because the paint is thicker than milk it wont froth up.