I have two simple rules:
1) Always start soft - you can easily overcall a close bird but it's really difficult to undercall any bird.
If there's no answer with a soft call, I will occasionally crank the volume with a friction call with little disregard. My opinion is that if the bird heard the soft call and was already on his way, then it doesn't matter if you crank it up because it usually won't scare him off - rather, it usually triggers a shock gobble from an otherwise quiet bird (and in effect lets me raise my gun).
2) Once the bird answers he decides every thing - use his responses to dictate whether you call soft or loud, fast or slow, passive or aggressive.
Caveat here is that I might not always be talking to him, but rather 'her' - the dominant hen who he's following. I've had great success ignoring the tom and cutting off the dominant hen, which makes her jealous and sends her on a search and destroy mission. On the other hand, I've also recently been quite successful sweet talking the dominant hen - you won't hear much about this technique, but (at least for me) it's been very effective in some of the regions I like to hunt...
And of course, always respect these two mantras and add 'when turkey hunting' to the end (like adding 'in bed' to the fortune from your Chinese fortune cookie).
1) Expect the unexpected
2) Anything that can go wrong will go wrong
S&R