Simple - very effective.
Potion of Deepholm - Item - World of Warcraft
1. Use it
2. Run to the portal that is there. (For example... Horde: Behind the center podium and to the right.)
3. ???
4. You're now in your major city!
Doesn't seem like much of a guide due to the length.. but think about how useful that is!
EDIT: Use it to bubble-hearth also