I know people that have worn their undersuits and such for that reason. Less bulk and weight in a case.
I did use a hard suitcase with wheels that weighed 4kg (9lb) empty. I then realised that i was losing 1/5th of my allowance just on the case. Now replaced with a simple zip up canvas dive bag that weighs about 0.3kg empty which is a big help.
Edit:- This bag
http://www.typhoon-int.co.uk/pages/diving/detail/id=48
Another useful tactic ive noticed is despite 1 x 5kg hand luggage a lot of airlines allow a laptop bag that isnt weighed as a 2nd carry on. So i take my laptop and fill its case pockets up with my dive computer, qualification books and other small items. It means i can at least double what im allowed to carry on with me. Not been caught *yet*