jam with other people, you can use their ideas and vice versa. I write some pretty good stuff on my own, but when I'm in a band I get inspiration from all corners. The way the drummer does a quick double hit on a kick can mean I change a riff, the bass player may want to add some slap, so you need to make room. After fiddling with things like that you find your songs sound much more original.
In addition, I'd suggest switching to acoustic guitar to write on if you play mainly electric, and vice versa. Using an instrument you would play differently, makes you think differently. You can still write heavy music on an acoustic, all you need to do is write it and transfer it to electric.
Experiment with different tunings, drop-d is the classic, it really opens up all kinds of different riff shapes.
Get a minidisc player, record a few riffs over it, or even chord progressions in keys you're unfamiliar with, try soloing using odd scales and chromatics. Everyone uses pentatonics these days. Try to break away from that.
Learn fingerstyle classical, check out this place often as people often name drop guitarists worth checking out that you may not have heard of.
Fiddle with guitar, amp and FX settings

There that should give you something to go off at least!