mix chords with scales, like play an f power chord then mix with some minor scale,
here are a few I use (the are all in the key of a, just move them up and down the fretboard to change the note)
e I-------------------------I
B I-------------------------I
G I-------------------------I
D I-----7------------------I
A I-----7------------------I
E I-----5------------------I
e I-------------------------I
B I-------------------------I
G I-------------------------I
D I-----------------------I
A I-----5------------------I
E I-----5------------------I
e I-------------------------I
B I-------------------------I
G I-------------------------I
D I-----------------------I
A I-----4------------------I
E I-----5------------------I
e I-------------------------I
B I-------------------------I
G I-------------------------I
D I-----------------------I
A I-----3------------------I
E I-----5------------------I
these are few, so experiment with scales and chords, minor scales are the ones for metal, but you can also use major scales.