I put 1 third water in (approx) then place the bag on the counter so that the water flattens out to create more surface area, then i pull the bag loosely closed opposite the bag seam lines and fold over the top twice.
Then holding it at the corners (opposite those bag seam lines) and kind of scrunch it inwards, letting a little air escape at that point isn't a bad thing.
Then you grab that scrunched bit with your left and twist the bottom of the bag with your right to make it nice and tight, then you put an elastic band around it twice. Then holding it up from the top in your left and the elastic band end in your right run it around the bag several times (this is really difficult to explain!) twisting the bag and band while you go, when you almost cant go around it any more at all, slip the band over the top of the bag again.
Oh, and they should be paying you. Your being taken for a ride.