In the old days you went to your local Chamber of Commerce, left a hefty cheque as a returnable security and they gave you a multi-page document called a 'Carnet de Passage' upon which you listed everything you were taking with you - electronic demo equipment worth several thousand pounds in my case. You then stood in line with all the lorry drivers at each country exit and entry point and got a Customs stamp to prove that you were entering and leaving with said goods. Two queues per border crossing. On your return, you went back to the Chamber of Commerce and, provided all your Customs stamps were in order, and all the i's were dotted and the t's crossed, they gave your cheque back.
The international commercial traveller community all did a communal dance when we joined the EEC and didn't have to do that for travel on the continent any more. Presumably you have to again now. Ah well, that's progress...