If you're in Prapat (Parapat), Lake Toba and you're heading south, but every travel agent says the buses are all full for the next few days (because of school holidays etc) they will suggest hiring a private driver (which isn't cheap).
The best thing to do is say that you'll sit in the isle on a coke crate. Yes, your butt will hurt for a 15hr trip to Bukittingi, but if you sit at the front you will have the best view, it's like sitting in the cinema watching a big screen with Sumatra going by and further on lots of people get off and most likely you will get a seat for the last few hours. :)
