If you're going to Pärnu or Kuressaare and the bus happens to be full, sit next to a hippie. They all get off in Viljandi and you'll have the whole seat for yourself after that. Peace!
When you come first time to Tartu and it happens to be with bus then don't trust your first impression -Tartu it's not like that. Some suspicious people just like to hang out around the bus station.