for Italy helmets must be ECE approved, DOT doesn't help. In Germany you may wear any sort of helmet as long as it is "for use on motorcycles", no special approval neccessary. If you got a navigation system in Germany use of speed alert data bases is prohibited, in Switzerland and Italy even the posession is - but you are not very likely to get checked for that. In Italy they got heavy fines for taking your hands off the bar or feet off the floorboards or not stopping at crosswalks. In Germany 5 km/h (according to speedo) over the limits is perfect (because speedos must read at least 5 km/h over actual speed) and 20 km/h over (actual) limit is still affordable (35 Euro in town and 30 Euro out of town). Switzerland and Italy are way more expensive. Riding under influence of alcohol (over than 0,05 %) is quite dangerous in all those countries, in Italy they may even confiscate the bike (for some time). Fluorescent vest are requested in Italy only for cars, not for bikes. Beware: in Italian traffic circles entering vehicles have right of way if not expressly signed otherwise, in Switzerland and Germany they do not. In Italy traffic lights don't show yellow before changing to red, in Germany ans Switzerland they do.
You may consider getting a one year membership with
Allgemeiner Deutscher Automobil Club as for just 100 Euro you get full protection for your whole trip in case of accidents, engine-failures, sickness or legal trouble (world wide actually). While Visa and Mastercard credit cards are almost everywhere accepted AMEX and Diners are not and there are still quite a number of shops and facilities that accept only cash (German police for instance

) so always carry some 100 Euro (Germany/Italy) resp. Swiss Francs.
Check carefully that your rental is expressly allowed into Italy and Switzerland and fully insured (especially against theft) for those countries (German term is "Vollkasko").
That's the most important things that come to my mind
