Whenver I am trying to play something for a while my right hand muscles start to get really tight and I start missing strings. It actually happens really soon like after maybe 15 seconds of playing.

Anyone got some tips on how to keep the right hand relaxed while you play?
Hmm that might be it, I just checked and I hold the pick in a death grip.

just keep playing and your hand will get stronger and itll stop
slash...he's all like... wow
Yeah i used to have that problem too it'll get easier. Eventually it wont hurt at all no matter how many times you play master of puppets lol. Good luck dude
If it still is happening and you are anchoring, try and stop using the anchor technique because that can sometimes build tension.
How hard you hold the pick will vary. Mostly harder for more volume which is
not something you're going to need much going through an amp.

In any case, most of the time I hold the pick with just enough force to keep it
from moving. This is actually fairly lightly even at high speeds.

If you find you're gripping it really tightly all the time, there's probably tension all
through your picking. Not a good thing.