Lots of people carry their mini-pumps in their jersey pocket. I prefer to keep mine frame-mounted (partly because women's jersey pockets tend to be smaller, partly just so I don't forget it).

However you carry it, I strongly recommend that you carry some kind of mechanical pump as a backup or a full-sized frame pump as your primary. CO2 is fine, there's no room on my bike for a full-sized frame pump, and it takes a very long time and some muscular endurance to air a tire with a mini-pump. But CO2 cartridges are single-use. Once you've used the one or two that you can carry, then what?

Same with tubes. Carry a spare. But carry a patch kit, too.

The other thing to remember is that CO2 escapes from tubes much quicker than room air. So the day after you've aired a tire with CO2, it will be nearly flat again. Some people let all the CO2 out of a tire as soon as they get home and pump it back up with air - I just keep it in mind so I don't assume I've had another flat when the tire is extra low the next time.