Leaf Blower
Gotta pass on my methods.
If the roof is easy to walk on (mine is), I use a leaf blower to blow out the gutters.
For my son's house, which is two stories and too steep to safely walk on, I put two 10 ft lengths of aluminum downspout together and put a U on the end with a piece of flexible plastic downspout, taped in place.
I taped on a thin stick about 6 ft long at the joint in the middle to stiffen it, and adapted it to the leaf blower. It worked great; best if leaves are dry. Tilt it a bit to kind of push the leaves ahead along the length of the gutter.
I described it to a relative who made one for himself and it worked so good that he collected $100 doing it for some neighbors.