Something I don't see mentioned and a thing which a buddy of mine does now, (& I use to do) is the shooting stance.
He stands with his bow arm, shoulders and feet all in a straight line to the target, having to turn his head almost 90 degrees looking right down his shoulder/arm. Once in a while even lines up with his back foot behind him a bit and really smacks his arm a good one then.
It's causing him to pull tight right across his chest and arm resulting in string catching a sleeve and/or trying to peel hide off his arm.
When I get him to move his back foot forward a half step, turning his body inward toward bow at roughly 45 degree into the bow to open up the shot, string slap is gone.
I still catch myself doing this now and then on a off-camber or sidehill shot at 3D target and I'll whack my arm or sleeve, throwing me off a bit.
Getting to correct 45 degree stance and opening the shot up a bit really helped me reduce string slap when wearing heavier clothes during late season hunts.......and helped my overall accuracy.
I have noticed before when turning in toward the target, that is my draw length is borderline long, it will show up here. going about 1/2" shorter (like I could probably do now myself) cleared that up
2009 Martin Bengal M2 Pro Cam w/factory STS & CCS.....66lbs, 29" DL, 422gr @ 272fps, Winners Choice string/cable, Trophy Taker drop-away Rest, Scott Release
1996 Martin Firecat XRG Pro Series w/Ultra Sonic wheels.....69lbs, 29" DL, 465gr @ 245fps, w/fingers & Martin leather glove