Q:

What's the best way to attach a hook to the back of my metal gutters? 

What's the best way to attach a hook to the back of my metal gutters?  The hooks will be holding some Christmas lights. 

A:

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.