Acetone, or gasoline (be way careful). and lots of elbow grease. I prefer the acetone (nail polish remover) just becasue I don't want to blow up.

I rub it on then get some scotch brite and keep going until most of the glue is off. Let it "dry out" for a few hours before you put new glue.

Yuck

Unless the existing glue is really dry, thick, lumpy or uneven there is no need to remove it as long as you are sticking with the same kind of glue. A layer of fresh glue will soften the existing stuff enough to even things out and allow the old glue to act just like the layers of new glue you would otherwise have to diligently apply.

Great answer!! I would do the same.
Dirtball!