I used 2 pieces of rubber band on either side of the main post of the button. Then I never removed the battery again. The rubber bands are just thick enough to push the button up against the battery and hold it in place.

FYI, the spring isn't soldered in or anything else. It is held in by the button. Originally your button would have had barbed posts on each side (so they would go in, expand and not come out again). Those have probably broken and now provide no protection. If you want to remove your old spring I'm thinking you would remove battery, use a narrow tweezer to squeeze the 2 side posts on the button in enough that they release. Of course, then it may explode (try keeping a finger on the button and GENTLY releasing) and you will have to find the spring again. Try doing it in a small area with a shoe box or something over the phone.