Ok, I have seen a few of these threads but, nothing like this..
First you have to complete the extinguishing the fires quest when the HH comes to burn the towns (I did this in Brill, not sure for alliance).
When your done you will get the quest to go into SM to kill the HH. (I did this at 70 so I'm not sure if lower levels pick this quest up also).
Now you can summon him over and over as long as you dont complete this quest Thottbot World of Warcraft: Quest: The Headless Horseman. You just have to wait till you can go back into the instance after the 5 times in 1 hour..
Ohh yeah, and with this you don't have to wait for a group of 4 others that can summon since you can do it yourself all 5 times..
happy HH farming