All of the advice given here is great, so just to paraphrase: the press roll has similarities with both e single stroke (it is always played with alternating hands), AND with double strokes (several bounces per stroke). A good way of practicing it is to start quite slowly, concentrating on getting as many bounces per stroke as possible. and I quote Joe Morello from memory: "eventhough the roll will not sound smooth at slower tempos, this will increase the eventual quality of your press roll".
So, you purpusefully place yourself at a disadvantage by playing it slower, only to then reap the benefits of your frustration later on, as you are able to produce a smooth roll at the slowest possible speed. This will make the faster executed rolls sound even better.
Quoting Jim Blackley: concentrate on making the buzz notes, played strictly in time, blend in with each other. I practice these on the pad, since this is a further disadvantage, so to speak: you will only hear the actual bounces. Now, when you move to the snare, you get much more mileage from the snares buzzing after the stroke.
Casper