Properly running "side stories" without derailment from main arc

To keep things fresh I like to have parallel smaller stories/adventures. Do you have any tips on how to run "side adventures" without derailing from the main campaign arc you are going for without directly entwining the stories. An example would be how would you sneak in a small planescape adventure into a mainly ravenloft story etc. Or maybe a small expedition out to the desert while the main story is taking place over in the sword coast.