I've just entered the 21st century, and bought a self-inflating sleeping mat. Can anyone answer my bone questions about how to use it? How long should I leave it to inflate itself, before I top it up? Should I top it up at all, or should it be a bit floppy? How do you store these things, rolled up in the bag, or opened out? I'm thinking about compressing the foam v. letting moisture in. Ta.