Hey there,
Practice playing to a drum machine or drum plug-in - it makes such a huge difference, much better than a metronome because you can add hi-hats & double bass etc between 'blips', syncopate your rhythms and do break beats etc.
The key to being able to play a kickass rhythm tightly is to know the rhythm in your head exactly before you try to play it. Sounds stupid but most people lose the rhythm because they lose the 'feel'. Try tapping the strumming/chopping out with two fingers (this works especially well because it imitates up down alternate picking). Before I started doing this I used to end up playing every bar a little different and the rhythm would go out all over the place.
One last tip, if you are playing a basic riff that repeats over and over, spice it up by changing every fourth bar (if its 4/4 time) - you can take this up a level by having rhythmic changes between these fourth bars to give a more interesting interplay. This really takes the monotony off a track that has very few riffs.
Basically, if you can't drum the rhythm exactly with 2 fingers on your picking hand then you need to re-examine the sequence and work out exactly what you want to play. The actual rhythm is the most important foundation to a great riff. The fretting hand movements are secondary to this. That's why I always suggest practicing metal riffs on either an open power chord or a riff with very few changes when either starting out or looking to advanced rhythms. For example, even if a riff has 20 different notes and plays over 4 strings, I will still practice the exact rhythm on one string to make sure I know what I'm doing before bringing in the fretting hand.
One last thing, palm muting on and off is extremely important - the best way to practice is trying those galloping rhythms and syncopated off beats.
Hope that helps!


PS..... everyone spells the word wrong, you included! :)