The most important thing you can do when practising any music, especially rhythm on guitar, is playing with a metronome or drum track (I prefer a drum track). Without a doubt, playing along with a drum track has improved my consistency, timing and general rhythm skills immensely. It's one thing to know which chords to play.. it's another to play them well and in time. A metronome is a boring way to practise I think.. but if that's all you have available, then you should use it!

For metal/shredding style of playing, you should scoop most (if not all) the mid-range out of your amp's eq settings... the hi-frequency end is where you mostly want to be with metal rhythm guitar but of course without some mid-range and a fair amount of bass, you are going to sound tinny. Also, most people prefer to use the bridge pickup as it lends more hi-definition response to the sound.. that is, it is less muddy.. and because you are playing distorted.. the last thing you want is to sound muddy..

When I am playing metal tracks, I tend to use 2-string power chords rather than full 3 or more string chords. I find that, unless I have the perfect EQ setup, the other strings just confuse the sound more than they help define the song. Since power chords are neither Major nor minor, you will need to use other instruments and chord progressions to help define the overall feel of the piece.. whether it is happy or sad (to put it simply).