I agree with Michael. Using a tray larger than the size of the negative is essential for even development.
I develop two sheets at a time, one up and one down. Every minute I pull them out of the developer, turn them over and put them back in, quickly. I believe this helps in completely changing the developer on all areas of the negative. I handle them in a different spot each time and do my best to keep them in the center of the tray.
Good luck!
Addition: Michael also makes a good point about choosing continuous or intermittent agitation. When I used Pyrocat HD I agitated continuously, now that I'm using Rodinal I agitate every 30 seconds.
Plus, if you start shuffling multiple sheets keep in mind your times should change as the emulsion will be against the back of another sheet for part of the time.