I'm a big fan of XLabs and their behind-the-saddle bottle mounts; but I'm sure most people who have mounted bottles there have at some point experienced 'bottle launching'. This is at best annoying, and at worst could totally derail a long distance nutrition plan. I'm aware XLabs sell a 'Gorilla' cage which they state will prevent launching, but they're very expensive and British road surfaces are pretty ingenious in that regard.
I've always used the cheap 'Elite' cages which are available in all sorts of colours, and have a rubber tab which holds on to bottles fairly well. However, after a recent launching episode I thought I'd try something new.
Step up, 80mm nitrile rubber o-ring. (I bought them from Amazon but I'm sure you can find them elsewhere). Wrapped as per the photo, they add a bit of friction to the cage and seem to work very well - certainly no issues in a recent race where I saw quite a few cages (Gorillas included) shedding their load.
Feel free to share any other top tips in the comments!