When I strum, I often have trouble with not hitting strings above and below the ones I'm meaning to hit, both with pick, fingers or sometimes the heel of my hand. I watched the Lamb of God "Killadelphia" and noticed for the first time how small their pick strokes were, so I'm pretty sure it's just that I have a loose technique in terms of picking, are there any exercises or tips I can use to tighten up my picking?
Keep it loose but make the motion a bit smaller, muting is also a huge part of it. Without a video it's kinda hard to make any suggestions.
Don't over emphasise the motions dude. Keep the wrist nice and relaxed and flowing, and keep the motions small. One of my first guitar tutors gave me the analogy that it was like flicking water off your hand, that kind of relaxed nature. Also a thinner pick apparently helps with strumming chords, I do see why but personally for me I don't notice any difference.