I write on the side of the cart with a fine point Sharpie, but I also have little 3 X 5 inch ziploc baggies that I keep all the carts for that particular flavor in since I used different PV's and different carts with each. When I want to change flavor, I just pull out the plastic box, look through the baggies until I find one I want, and switch it out. I only drip to test new flavors and I don't like filling and messing with cartomizers much, so I fill a bunch of them at a time (ones that don't leak) and that way I'm set for days. I haven't had any leaking problems yet, but if I did, they're pretty protected between the plastic baggie and the plastic box, so nothing should get ruined. Also, that way, when I have to run out somewhere, I'm never late because I was filling a cart. I just grab a full one from one of the bags and go.
I don't like surprise tastes, especially first thing in the morning, so at this point, this is what works best for me.
Good luck!