Tundra_Man 

As CBadRider correctly advised, your best bet during a tri is to spin your feet faster in a lower gear versus standing and cranking a harder gear up the hill. Standing and cranking may actually get you over the hill faster, but it will tire you much quicker. You've got a run waiting for you after T2 so you don't want to leave it all on the bike course.

A good way to learn how to spin is to spend some time on a quiet side-street or parking lot and put your bike in a really low gear. Use a watch/stopwatch to figure out about what 90 RPM feels like. Then practice spinning that fast (or even a bit faster without getting ridiculous.) Don't worry about how fast/slow the bike is going, just focus on spinning the pedals. After a while (probably more than one session) you'll learn how to spin smoothly and with less effort. At that point you can begin doing some training rides up hills and learn how to find the gears that allow you to spin a fast cadence to power you over the hills.

As an alternative to the stopwatch, you could buy a fancy bike computer that includes a cadence sensor and use that to figure out what 90+ rpm feels like. A bonus to having a computer is that you can also use it during a race on hills to make sure you're spinning rather than mashing. I have one of these computers on my tri bike, but once I learned how to spin correctly I rarely use this feature so I'm torn on whether or not it's worth the cost over a standard computer. Once you learn to spin I really haven't found the cadence sensor to be of much benefit for anything else.