first thing i thought of when reading your questions was the standard drill of opening the door and jamming something in it(your girl friends shoe my instructor allways said!)when you did all those PFL's in your training!Second is, when ditching in water i was taught,theoretically of course,to read the waves and land along the wave in the ditch as this will increase the chance of a flat arrival,also keep pulling on the nose untill you stal almost in a tail slide,practice that one up at 4000ft if you like!!When crossing water allways use the highest possible altitude available,7500' over the channel now! Allways keep a mental picture of the wind and also a good look out for shipping,there is a lot of it in the channel,and landing along side one of those might be a god idea!
But surely a thorough pre-flight is the most obvious precaution!
Safe and happy flying!