Don’t let them with batteries for an extended period of time. The batteries will eventually spill their chemicals, ruining the contacts of the calculator. This grit is extremely difficult to get rid of - and in some cases you will have to change the contacts (see our repair kit for battery contacts for the HP41c). It is a pity that many calculators cannot be repaired that way.
Do clean them regularly. I use a computer cleaning liquid, but I have heard that lens cleaner works well too. Don’t let grit take roots on it!
Don’t use cyanoacrylate glues when you need to fix something: they dissolve the plastic, and it will be much more difficult to put it back in proper shape.
Don’t operate HP models that have battery and power supply without the battery! It acts as a filter, and in some cases as a rectifier - you will surely damage and burn your calculator if you do it for any extended period of time.
Do carry with you a fresh battery set when traveling - some calculators have a limited period from the moment they show the warning sign until they give their last breath - and this period gets shorter over the life of the calculator. Some of them (new HP15c LE) do not signal at all the low battery condition!
Please write to firstname.lastname@example.org if you have other dos and don’ts to add!