You know those little "0" shaped stickers you put around the holes in punched paper to keep them from tearing? Open the slide on the phone and turn it over. Put some stickers (2 stickers thick, one on top of the other) on either side of the "divots" alongside the black square (that is the back of the screen).
Let the stickers slightly overlap the "divots" so that they fold over into them, and use your fingernail to tuck edges into the divots. (These stickers should be positioned near the top, meaning furthest away from the body when the phone is open.)
Next, put a couple more stickers, 2 or 3 thick, in the same way, as near to the body of the phone as possible.
The added bit of thickness behind the slide, and a touch of friction in the divots that the top uses to slide open in the "open" and "closed" positions makes such a difference! Now the phone feels solid when closed, and nice and stable when open, without that annoying "play" in it!