I usually hit anywhere from 85-92% depending on how much grain I have in my 10 gallon Rubbermaid tun. I observed the highest efficiencies on seven to ten gallon batches using a step mash with around 17-20 lbs of grain using a modified batch sparge. And my final sparge time rarely takes more than 15 minutes with no ill effect on efficiency. I do the following and it seems to work well for me...
1) I design the volumes of my mash steps to a ratio of 1.75 qts/lb. to help ease conversion
2) I always use rice hulls.They not only eliminate channeling, but a thick layer at the bottom acts as a filter
3) I always stir the hell out the mash, rest and vorlauf before draining the tun
completely between steps
4) I also always sparge in a similar manner... by filling the drained grain-bed with just enough sparge water to set the bed in suspension, then stirring the grain bed and utilizing a 5-10 minute rest to settle the bed, vorlauf, followed by fly sparging with an increased flow rate. IMO, doing it this way helps get the remaining sugars in suspension, and the faster sparge rate flushes the sugars out of the bed.
While I have only obtained 95% mash efficiency once, I repeatedly obtain 100% mash efficacy.