1. Down pick, down pick, down pick. No matter how fast the tempo, always try to down pick the on-beat notes. Not only does it sound better, it also keeps you in rhythm better and keeps your technique suitable for syncopated beats. James Hetfield is an absolute god at this. It's a sorta rite of passage in the metal community to be able to play Master of Puppets all down picked up to proper speed. A forearm killer but absolutely worth it in terms of developing proper technique.
2. Practise at high gain so that your muting technique develops proper. This is contrary to popular wisdom that you should practise clean. It took me a long time to come to terms with the fact that my perfect classical and jazz technique sounded god awful as soon as I added more gain as all the unplayed strings were constantly ringing out and muddying the sound. You need to be muting all the strings you aren't playing, with both hands. Also you need to make your palm muting technique sound good when playing riffs.
3. In general, bridge pickup for rhythm, neck pickup for lead. Cut bass. Exceptions of course.
4. Metronomes. Also recording double or quad track parts to see how tight you can get it.