Currently I use 2 (short) bottles on my frame and an aero bottle, but I'm thinking about getting one of those refill systems that sits in your frame and has a drinking tube that winds up like a camelbak. I am afraid I may still want separate water from sports drink... so I'd probably use that for water and bottles for drink (I can't fit the on-course water bottles in my cages), or the aero bottle and that (which seems kind of silly), or the seat post mounted bottles which does seem interesting. I guess my seat post isn't exactly... round... but I'm sure that's not a huge issue.

I'm carrying my tools (cartridges, tube, patch kit if desparate) on my seat post now but some of those (nicer) seat post mount systems have a place for that stuff, too.