This might be a silly question, but what sort of trousers do you wear when riding your bike?It took me a great effort to find something that worked for me. This isn't like something that's readily available even at Giant's store. I took my bike in a few times for this issue already since they wanted to take a look, every time they said it's taken care of, I was able to take the chain off minutes later just by going over the speed bump in their parking lot. I showed them how it happens, they wouldn't care. All they claim is it's normal. How is it normal if I cannot ride the bike without the chain coming off almost every ride?
If there *was* a simple and cheap solution readily available, I would've been very happy.
There has to be a reason why your chain keeps coming off. Is the derailleur putting the correct tension on the chain? (You said the chain comes off mostly in the higher gears, or small cogs where the tension is lower).
I am sorry you're having this trouble, it must be really frustrating for you.
IT IS NOT NORMAL for a bike to do this.