* Check out the museum's strengths: local artefacts, the Roman age, pottery, steel, documents, whatever they are try to capitalise on them; see in particular what relates to the periods in history your group is studying.
* Visit first: preferably with three or four pupils, so they can report back and appropriate preparations can be made; this creates a feeling that pupils themselves are involved in the event, and it is not merely something unpleasant being done to them, like extracting a tooth.
Even if there are interactive exhibits, compile a list of things to find out: some pupils like to pretend they are detectives (others are not so easily fooled).
* 'Hands off' is a strict rule: it also avoids the embarrassing problem of needing a whip-round to raise the odd grand for a broken artefact.
* Get children to make (polite) suggestions for improvements in the museum after they have finished their visit, like the need for a bit of interactive technology and hands-on features.