I've had pretty good luck using both heat and pressure.
For the heat I basically do it the same as I would for opening the phone; a minute or two with an iOpener, or else get it nice and toasty with a hair dryer. I don't use a heat gun any more, not since I destroyed a Samsung LCD by getting it too hot.
Once the adhesive is heated, I lay it face down and set an old motorcycle battery on top and leave it there overnight.
Obviously there are any number of variations on this possible like using screen clamps, etc., but you get the idea - soften the adhesive then put weight/pressure on it to seal it as it cools.