Warcraft III/Routes

Generally, go watch Liderharens Videos

Human X02

In human X02, buy 2 scrolls of tp and go to one of the outer bases, get the gold and tiny castle, use sorc invis, invis up to the undead base, and place a town hall in the left of the undead base, where the undead don't aggro too. Then let Kael die, he will respawn by the Naga, and you can all teleport in.

In the bonus campaign, specifically when rescuing Bairne (it works elsewhere but I don't think it's useful)

If a missile is in the air (such as rokhan's glaive) and is about to kill a hero as a cinematic starts, the hero will die during the cinematic and will respawn at the ordinary spawnpoint. This is useful when the spawn point changes automatically after a required cutscene and your heroes get teleported around, so that you can teleport to a specific spawn point. For example, when saving Bairne.