I had a toilet that was always leaking because the floor was uneven and no matter what I tried the bowl rocked a little and broke the seal. I eventually re-tiled the floor but in the meantime I used one of these rubber seals and it held for a number of years.
http://www.supplyhouse.com/Sani-Seal...SABEgKJ3fD_BwE
similar type used in RVs, because of the movement