Use a carry on for your equipment
I always take my equipment as hand luggage so it’s with me all the time. This might be obvious, but I use a cabin size bag and I check the airlines website beforehand for the dimensions. When I get to the plane, I always make sure (where possible) that my bag is above me. I also watch it like a hawk to check no-one moves it (haha I sound crazy)…
Back up to a hard drive or cloud storage (if your wifi is good)
After I have photographed the wedding and I get back to the hotel, I always do a back up to a hard drive (and cloud storage/gallery if the wifi is quick!). I then put the hard drive and memory cards in two separate places.. or two separate rooms if I am with someone with another room. If there is a safe I use that, and/or put them in my suitcase and lock it whenever I go out..
I usually feel better booking my flight home the day after the wedding, so I can get the cards home safely. However, I know that’s not always possible. Plus I like sightseeing!
Keep cards/hard drive separate on the flight back
On the flight home, I always keep my memory cards and back ups on the hard drive in two separate places. So either handbag/bumbag and my suitcase trolley hand baggage (not my check in bag). This way, if some crazy situation happened where I lost one item, I have another copy of the photographs somewhere else!