Bernie, I like to grind and tickle the horns vs smashing. I start with about 30 sec -sequence but then drag it out to 45-60 secs per and on the half so every half hour. I also find grunting prior and just after the sequence and even sometimes at the 15 minute interval will help in keeping their attention. I always rake trees and ground prior and after. Also like to rustle leaves with my boot heels while rattling and end with me thumping the antler bases on the ground to simulate a deer fleeing. my grunts I vary to soft low toned contacts, tending grunts and even aggressive low toned drawn out grunts can produce. I have used doe in estrus scent to spruce up the set up as well.

Some things to consider is if you are trying to lure a animal out, think of a spot that would be comfortable and natural to them or a fight. I have never had good success with horns and fields or food sources. I much rather rattle in transition area, draws, rublines, meadows..etc. So where the deer like to move and stay concealed. I also pick areas with timber ridge to rattle from, deer like to move in low areas and they tend to do battle in these areas as well. I have been witness to a few scraps, most of which were in low spots, bowls, hallows,etc...never seen 2 bucks go fully at in the open. Oh I have seen them run each other plenty in the open areas, but never do all out battle and lock up. In my mind location is a big key when rattling, pick a comfort zone for the deer and you'll find your success rate will go up. Also always have the backdoor covered, mature bucks will often circle the fight or your position and try and come in downwind...so always pay attention and make sure you have a shooting lane out back. I thing wind direction is another key, I always hunt with the wind on my nose and rattling is no different. Start of soft and short, then gradually bring up the tone and intensity and always keeping contact buy way of rattling or the grunt/doe bleat. He may be coming and then hear nothing drop his attention level and he is gone off in another direction. (Don't over do it, but if you stick with 30-60 sec sequence every 30 minutes and call at least every 30 minutes or at most every 15 minutes, you'll be fine) I started a rattling log a few years back in which I record the hunts events, it has proved useful...so I would suggest trying that and let the deer tell you what they like.

Good Luck!