As far as how I keep my over healing to a minimum, I always cancel healing spells if the target gets healed to full or almost full before the cast lands. Of course, all healers do that. I'm not really sure how to explain how I get lower than all or almost all healers, though, because I do not personally know how other people heal. I think explaining my techniques may help explain how my overhealing is lower than normal.
I look at healing as a competition, so I heal aggresively. I fight for landing the first heal when I can, but always make sure I save the mana if someone else beats me. I don't use healing meters though, I never think that the best healer is the one who tops the meters, or even the one who has the lowest overhealing. Generally, for trash, I try to keep swiftmend on cooldown each time it comes up and use natures swiftness when I think it is necessary. Sometimes that is harder than others when you only have a split second to make your decision. Regrowth is my favorite tool for trash healing. It has a 2 second cast, and crits half of the time if you have talents in it, so it is awesome for a fast, big heal, AND you can use it for swiftmend. The drawback is that it costs a lot of mana for one healing spell, but generally that won't matter because we can drink right after the pack is dead. In some cases though, (Hyjal Trash for Az'galor) we can't drink, so I try to limit my spamming when possible. When people only need to be healed for a small amount, I use lifebloom because its cheap, heals fast, and if they take another hit of damage, I can throw on another stack or two to help heal them.
I try to "race" other healers to a damaged target. Like I said before, just because I heal aggresively, doesn't mean I try to win at meters or anything. But this racing helps heal the raid faster and maximizes my healing effectiveness in my opinion. If we are in a situation where people are taking spurts of damage, like 3k or so, and one person takes damage first, then others begin taking damage almost immediately after they do, I will often stop healing the person who took damage first to heal others that just took the damage. The reason I do this is because I know some healers are already casting heals on that target, and there is also less competition currently on that freshly damaged target. I often use natures swiftness to beat other healers to my target and throw up rejuvenations on raid members to swiftmend them. I do not normally use rejuvenation as a healing spell for trash unless I want to swiftmend them or I want to put it on the tank so I can instant heal him if he takes a burst. There may be other rare occasions that I use rejuv, like when I know other healers are not going to be healing my target and I am sure he or she will be fine with a slow hot ticking.
Boss fights are usually not the same technique as I use for trash, I don't think there are any two bosses where I do the same exact type of healing. I do whatever I am assigned to or needed to do.
I think my style of aggresive healing is very effective. Of course, I don't purposefully go out of my way to top meters, and as I said, I don't use meters of any type, and I don't get argry if I ever don't perform well compared to others or if they are catching all my heals before me. Hopefully that explains how I manage to have a low overheal compared to others. If anything is unclear or if anyone has any questions about how I heal, feel free to ask, I'm sure I made a large handful of typos and had several ramblings that didn't make sense in this gigantic wall of text.