Eric,
Are those who are working AO-51 mode L/S mostly using rotors and tracking
software?
Yes, though in some cases the rotators are "Armstrong Rotators." That is, hand-held antennas.
What are you using to keep the antennas pointed for mode L/S and what are
you using for antennas?
Any AZ-EL array will work fine. The beam widths are not that narrow. For the S-band side, most people use a small dish, though AO-51 is so loud that even a modest Yagi or loop Yagi is fine. For the L-band, a linear Yagi or a helix. NOVA, SATPC32, Ham Radio Deluxe, or something like a TrakBox will easily handle for pointing the antennas.
At these frequencies, the real need is for computer frequency control to compensate for the Doppler. It can be done manually, but if you don't have that, you are going to be spending a lot of time talking yourself back on frequency, and giving and asking for repeats. SATPC32 of course works perfectly, and the new, improved Ham Radio Deluxe has performed well.
Alan WA4SCA