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. On a dry day, use the mirror on the Gutter Viper to check if the gutter even needs cleaning.
But you can do something else 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. You can also use pop rivets, but be prepared to drill these out and replace them if you need to take the elbows apart one day. That is a simple task. However, if you keep the gutters clear, you'll probably never have to do this.
If you must use screws, use the shortest screw one practical (say, 1/4"-long) and place it on the top surface of the second elbow, not the sides. Also, consider snipping or filing or grinding off the sharp point (before insertion) to decrease the risk of its snagging debris on the inside of the elbow. Repeat for the next elbow.
Instead of using the traditional two 90 degree elbows, you can instead use two 45 degree elbows separated with a short length of downspout in between. This setup increases the speed of the water and debris flowing through those elbows, thereby reducing the risk of clogging.