3" shells are a good compromise. Will get you distance but are less harsh on the shoulder. Plenty of good turkey loads out there in 3" shells.
Box, slate, then diaphragm. Take up calls in that order and practice. I'd advise getting a tape or CD of live turkeys. Listen and try to emulate different scenarios/calls (eg. fly down, clucks, yelps, etc.) Once you get pretty decent with the mouth call, don't hesitate to take it out into the woods. A mouth call can get a bird in close when you're frozen, whereas you have to move to manipulate a box or slate.
Locator calls are good for roosting as well as for locating early morning when birds come off roost. I haven't had as much luck with them on Eastern turkeys in the mid-day though.
Pattern. It would be silly to go into the woods without an idea of your pattern and subsequent effective distance. Open distance call fool you, so sometimes it pays to step off effective ranges if you have the luxury. Last year, I took a shot out into an open pasture that I would have sworn was 40 yards or under (I had my gun effectively patterned at 50 yards with Hevi-Shot). Turned out to be about a 60 yard shot. Everything went low, and that turkey flew away with his life.
Be still. Don't squirm, fidget, or blink too much. I think this advice is the most important of all.
It ain't always like the television shows where birds announce themselves and come in strutting. I've killed birds that snuck in without a peep. After gobbbling up on a ridge, one older tom took over an hour to travel about 100 yards, and after those first gobbles, he never made a peep. Sure enough though, he came sneaking down from my left and finally wandered into range.
Killed another bird that I simply heard scratching leaves when I was walking down a pipeline cut-through. I hunkered down out of sight (he was below me on a fairly steep incline) and just waited. And waited. He eventually walked uphill, popped out into the cut-through, took one look at me, and flew. I shot him on the wing at about 15 yards. Good eating.
These two kills remind me of another point. Turkey time is a thing unto itself. Note that they don't carry appointment books or palm pilots.
Seriously, the best way to figure out what works is to tag along with someone who is good at calling birds and then trying to mimic what they do when you practice.
