Roll of 3/4" plastic electrical tape
Flatten the bag on a flat work surface. Start at the closed end and fold over a 4" section. Continue folding sections until you have the entire bag folded into a 5" x 22"stack.
Hold the folded bag down so it doesn't get away as you fold the right end over to the left end. Fold the stack over again and tape it in two wraps of electrical tape.
The empty bag, which originally held 50# of sunflower seeds started out at about 36" x 22". Wadding it up and stuffing it in the trash would take quite a bit of space. Folded and taped it is about 6" x 5" x 3".