While routing there is inevitably the moment where you say "crikey...I can't go that way". Alas, there is a solution that doesn't require you to stop and delete a bunch of stuff one at a time (you could but I'd ship more products than you if you did :). Instead, just hit Backspace and it'll go back to the last point in which you clicked (the last anchor point for the track / wire). You can do this all the way to the start of the active route. If you need to later break wires, have a look at my tip on breaking wires without deleting the whole thing.