Jagwire inline cable adjusters are the answer. Fairly common on cyclocross bikes, where you may well need to tinker occasionally while you're riding. These are MUCH easier to deal with than adjusters at either headtube or downtube stops; just park 'em out front about a couple of inches from your STI levers, or tuck 'em in near the stem if you're running Campy.

http://www.jagwireusa.com