use the shifters as a guide as stillriding says. right-rear, left-front. this way all you motorcyclists (typically they swap levers for brakes, except for Harleys which are reverse of the reverse...), Italians, lefties, and people from India (as a friend from India pointed out after he crashed into an office chair riding my bike around the office) can signal the same way.
of course, if it's a front and you're about to crash, worrying about what hand to use to signal is the least of your concerns. I was in a road race in NY when someone flatted on a 45+ mph descent. A hand went up and I thought "oh, he knows how to signal a flat". Then it zipped out of view, I heard the scraping of metal and ground, and the next thing I knew about 20-30 racers were sprawled all over the ground including me.
For the original questioner, if you raise *any* hand, hold your line, don't fall, and manage to filter through the pack without taking anyone out, then you've done as well as anyone would expect you to do.
btw no matter what wheel you need, the neutral support guy will probably ask anyway. When my dropout broke in a race I stopped without really signaling but it was apparent it was a rear wheel problem since it wasn't rolling at all. "8 or 9 speed?", the excited neutral support guy yelled. "10 speed. and I need a dropout". "oh". "can I at least get a ride? it's 10 miles back to the parking lot". "um.. ok".