Allowing the gutters to fill with debris increases the risk that downspouts will clog, so keeping the gutters clear is the first step to preventing downspouts from clogging. But you can do something to decrease the risk of a clog. Galvanized gutter screws are often used to attach the first elbow to the gutter, the second elbow to the first, and the downspout to the second elbow. While these screws are convenient because they are easy to put in and take out, they increase the risk of clogging because their sharp points snag leaves and small branches. To eliminate this risk, use a pop rivet to attach the first elbow to the gutter - its lack of a sharp point means it will never snag debris there. You can also use pop rivets to attach the rest of the pieces, but be prepared to drill these out and replace them if you need to take the elbows apart one day. This is a simple task. But if you keep the gutters clear, you'll probably never have to do this.
If you must use screws, use the shortest one practical and place it on the top surface of the second elbow, not the sides. Also, consider snipping off the sharp point (before insertion) to decrease the risk of its snagging debris on the inside of the elbow. Finally, you can do the same to the screw holding the top of the downspout to the second elbow.