The problem appears to be when air gets into the tube of glue - if air is present in the tube, evaporation will take place. This air can enter the tube during patching, or from a poor seal between the cap and the tube.
I was able to get a tube of glue to last for over 2 years (and 8 patches) by following advice from the other answers here:
- Make the smallest possible puncture in the tube's foil when first opening the tube (I used a pin)
- Apply the glue and re-cap it in a single action (i.e. never release pressure on the tube, as this will allow air to enter the tube)
- After using the glue, always hold it vertically and squeeze out any air
- Squeeze out sufficient excess glue to completely seal the edge around the cap
I wanted to use the glue again last night (9 months since I last used it), and it was no longer usable, but since there was liquid glue in the tube when I last sealed it, it probably would have been usable for at least another 3 months.
One problem I experienced is that the cap split on the fifth reseal - this may have been because tubes aren't really expected to last that long, or possibly because I was being too forceful screwing it on. In any case, the excess glue sealed the split.