There are very easy rules you can follow to achieve a great muting technique.
1. The part of your hand that you use to palm mute can make your muting most effective. The two best ways are with the side of your hand that runs down from your pinky finger, and the meat of your palm that is attached to your thumb. Both of these techniques should be used, so get comfortable with them.
2. Where you place your palm on the strings is the most important factor in muting the sound versus killing the sound. You must lightly touch the strings right where they come off the bridge. If you move too far towards the neck the sound will become deadened.
3. Your amp setting can be useful in making a cool muted sound. Although you can get a muted sound with a clean amp setting, if you add distortion to your sound you can really hear the technique jump out.
4. To create a dead percussive sound as an effect you should use your left hand touching the strings over the middle of the neck.
My advice is to start with these techniques and experiment with getting your own unique way of muting. Metallica, like many heavy bands, uses muting to enhance its overall sound so use this to create force and have some fun!!!