Great question — answering as the UphillTech team:
They sell clips like this at the Home Depot , which can either clip under your singles or to the gutters. Not sure how well they work yet, I picked up a box and plan on giving them a try this year.
Full answer by Tester101: https://diy.stackexchange.com/a/10432 (CC BY-SA 3.0).
Need it done for your business? UphillTech.
