I find nitrous is wasted if you keep your finger on the button while shifting gears, so while I used to use index and middle fingers (R1 & R2) to control upshift and nitrous, now I just move my index finger as needed (i.e. release nitrous to shift; when the next gear engages, back on the nitrous).
I don't know if it is just my imagination or not, but the nitrous seems to last just that little bit longer if I do it this way. I like my nitrous... and I like it to last. Hey, 340kph down the back straight of Nurburgring in a Subaru Impreza could only be done with the happy blue bottle. And besides, it's a pearler for getting out of tight corners real quick - nose aimed at the exit point, up a gear and hit the spray... Whee!