Mount up, dismount.
You may now Sprint without loosing a single point of stamina.
(As soon as you use any attack abilities your sprint starts costing stamina again so you'd have to talk to an npc again.)
(Simple bug, but very nice to know since dismounting takes a very long time, and you can dismount instantly by talking to an NPC, so sprinting is generally faster because of this, and easier to control.)
NOTE: Only works with the level 40+ mounts, NOT with Rhino/Mammoth.