The heels of both hands for standard sweeps but I also picked up a nice tip from Theodore Ziras: As you sweep down (low to high E), mute the strings you have yet to play by not fully fretting them with the right hand and mute the strings you have just picked with the side of the thumb gripping the plectrum. Hard to explain without a photo, hopfully you get the idea... This really helps to get articulate sweep as it's not easy to fully mute some sweeps using only the heel of the right hand. Hope that helps.