Have you tried out our custom iPhone 3G/3GS front panel adhesive strips? They are guaranteed to keep the glass in place and include the free repair guide.
Hope this helps!
Custom-cut adhesive strips for installing iPhone 3G or 3GS Front Panel — 50+ available at 2.95 each.
It's very important to make sure there is no foreign material (ie glass pieces, old glue ball, etc.) on the plastic. This will keep the glass from seating compleatly.
It is also very important that you do not touch the glue strips as the oil and dust on your hands will diminish the holding power of the glue strips.
Lastly, be sure you apply pressure for at least a minute to each of the corners so the glue will get a chance to adhere.
The glue strips that are sold by iFixit are great and I have never had a problem.
I don't know what type of glue the strips are made of but I have experience with 3M VHB (Very High Bond) and it takes a couple of days to reach it's full strength.