I think most people on this site know about tapping. It's been discussed quite a bit in the past, but those posts have gotten buried over time. There is also the method of grouping at least two lowbies, then using ungrouped high-level pets (warlock or hunter) to do the kill. With this technique, level differences don't matter between the lowbies and the killers, the lowbies get full XP (shared between them of course since they are grouped).