okay, if you more or less have palm-muting (and it's like a bicycle - you never forget how) then perhaps we can move to playing actual riffs.
I forget what this is called, but this is the basic template for a lot of metal riffs:
2 strokes on the low e (open), palm muted, then 1 somewhere on the A string
like this:
----------------------------------------
-----7-----9-----10----9----------------
-0-0---0-0---0-0---0-0------------------
and then practice practice practice those chugs
this is the opening to Amon Amarth - Pursuit of Vikings:
0-000-0-000-000 (listen to the song for the timing)
Easy as fuck.
Then the next bit is:
----------9------------------
-7-9-10-7--------------------
------------0-0-0-000-000----
pretty simple. You just need to be able to do the three chugs there. Keep practicing.
Now then, other techniques:
Hammer-ons and pull offs.
Basically, you play a note and while still holding it slam another finger down on a higher fret on that string, causing the note to change. For pull-offs you do the opposite. Very simple in theory. If you have a shitty guitar these may not work to well. Practice is key. Mostly for soloing. But some riffs use them. This is one that I wrote (so please don't steal it for a song).
------------------------------------------4------------
-------7(10)7-------5(8)5-------3(6)32(5)22------------
-0-0-0--------0-0-0-------0-0-0------------------------
(10) means you do a hammer-on to that fret. Take this nice and slow.
Slides
A slide just means you play a note and while it's still ringing move to another fret without lifting off the strings. Very useful, and you can do it with single notes or chords. Great for thrash riffing. Again, my riff (that I might actually use), so I'd appreciate not hearing this on anybody's album.
------------------------------------
------7/5------5/7------7/6/4/66----
-0000-5/3-0000-3/5-0000-5/4/2/44----
Bends are pretty sweet as well. You basically while fretting the string move your finger up or down (i.e. towards you or towards the floor), thus changing the pitch (upwards, always changes the pitch up). I'm not too great with these myself, and they have limited use in riffs, but they're the heart and soul of lead guitar. Check out Celtic Frost - Procreation Of The Wicked for a riff that uses bends.
Now then, tremelo picking. If you took my advice you've tried alternate picking. Tremelo picking is pretty simple: stroke up and down really fast while holding one note. You can change notes, but unlike in shredding you play each note a bunch of times really fast. Amon Amarth do this ALL THE FUCKING TIME, as do a lot of melodic death metal bands, really.
Try this:
---------77777777777888888888877777777------
-55555555-----------------------------------
--------------------------------------------
btw, you'll notice that most of these riffs aren't complete, in that it sounds fine if you play it once, but isn't interesting played multiple times. There are a couple ways of dealing with this:
1. Play it the same way three times and then do something different the fourth
2. Play it two different ways and alternate
3. Play it twice then switch riffs (which has the advantage of making you seem like a better guitarist even though it's actually easier

)