From the experience of this task giving me herniated discs and preventing me from ever bending over or lifting heavy objects without extreme pain. I am able to share this knowledge. The best method which requires the least amount of force is accomplished by twisting the manhole cover. When the cover is off there is an unsafe rare occasion when the small crowbar hole is in the closest position near the manhole. This is when you will be tempted to pull the cover in a straight direction without twisting the cover. This is what caused my herniated discs since this method requires the most amount of force to move the cover. Instead of pulling the cover in a straight direction you should make two movements by twisting it off to the side and then twisting it back into place. This knowledge should be shared with everyone who must move manhole covers!