Okay, I'm trying to learn how to play Hammer Smashed Face by Cannibal Corpse on guitar. I have the intro down so far. Then the verse riff comes that goes like:
eb----------------------------
Bb----------------------------
Gb--------4-------------------
Db--2-------------------------
Ab------------2-3-------------
Eb-----------------2-0-1------ ad nauseum

I'm finding it very hard to pull this off cleanly. Also the alternate to this riff that comes later.
eb-----------------------------
Bb-----------------------------
Gb-----------------------------
Db--------3--------------------
Ab---2---------2-3------------
Eb-----------------2-0-1------

If anyone knows how to play it, can you tell me of any exercises or anything that helped you master this riff?

Any string skipping exercise should be sufficient, you just need to get used to moving your fingers and picking in different ways and accurately hitting the notes.

Also, you seem to be missing a few notes there, check a tab site for a tab, just make sure it sounds right when you play it.
http://www.ultimate-guitar.com/tabs/c/c ... r6_tab.htm

I have the Guitar Pro Tab from UG, Version 8. It sounds right to me?

eb----------------------------
Bb----------------------------
Gb--------4-------------------
Db--2-------------------------
Ab------------2-3-------------
Eb-----0-----------2-0-1------


eb-----------------------------
Bb-----------------------------
Gb-----------------------------
Db--------3--------------------
Ab---2---------2-3------------
Eb------0-----------2-0-1------

You forgot the open eb in both parts of the riff.As far as I remember none of the notes are palm muted so you have to mute the tonic notes as soon as you pick the a# in the first part aswell as the e in the 2nd one.

As far as for learning how to pick the notes,if you have amazing downpicking speed and control play the riff that way,should be the easiest way for you that way.However,if your not quite up there then you have no other choice but to use alternate/economy picking,whatever the picking style is you are used to.

Your goal should be to be able to distinguish and feel each note you pick,while not putting much of a strain on either your right nor left hand.So the first thing you would want to do is that before you mess your technique up by constantly missing strings/hitting dead notes/having unwanted open notes ringing etc is to find your comfortable playing speed for that riff,which shall neither be too fast nor too slow for you.Play that riff at this tempo until you feel you can play it through about at least 4 times without mistakes and then raise the tempo by about 2-5 bpm.Repeat that procedure over and over again until you reach the 200 bpm mark or whatever the speed that riff is in.