Watch the Stella by Starlight online guitar lesson by Mimi Fox from Jazz Performance
Many students are perplexed by fast double-time lines and wonder not only HOW to play so fast, but how to CONCEIVE of the lines so rapidly in a playing situation. Here's the deal: in my own practicing, I play VERY slowly so that I can work on clean execution and complicated figures with the metronome clicking slowly. This gives me plenty of time to shape my lines and plenty of time to think. Gradually you can increase the tempo and the lines will start coming to you faster as you do this all the time. Be patient with yourself and don't increase the tempo until you can play cleanly and intelligently at whatever speed you are currently at.