Mine are normally around 2-3 mm (leafhoppers, psyllids, etc.), but for working material I tend to use a basic water-soluble glue like Pritt - the type that comes in sticks. You don't want anything too liquid, as it can easily envelop and obscure small details like setae. I also tend to arrange them on points rather than wide cards, so that they can be mostly visible from all angles without re-wetting. Although the water-soluble glue makes it easy to adjust, the more manipulation that is done, the more chance of some damage.
What I don't know, however, is how good Pritt is for long-term (decades to centuries) storage. I've seen no problems for specimens kept a few years, but can't guarantee the 'archival' standard. I'm sure others have more experience...
If storage is proper, I see no reason why it wouldn't hold long term, if it had held for a couple of years, as you say. I will try that, thank you very much!