Practice with the basics.
First use cardboard, posterboard whatever is handy and get your paint/thinner ratio and air paressure adjusted so it's flowing good and smooth.
Then practice the basics. Paint lines getting a smooth start and finish, do different widths get steady doing that.
Then practice the dots, shoot a line of dots all the same size and try to keep them consistant. Do line after line of different sizes and coverage.
It's all about getting consistant with your airbrush and making it do what you want it to. Then you have to transfer that technique to a model and you may require some changes on paint / thinner ratios for the plastic but the applications will remain the same.
Here's a link with some of the practice exercises that will get you back in shape in no time.
http://www.howtoairbrush.com/Basic.htm
Eric
Good luck and remember, practice!