I have 3 songs on the go, all have have two guitar parts to (lead and backing). My problem is for two of them, writing a chorus riff and the other a pre chorus riff, but im totaly stumped, i cant seem to write anything that works.

Any help would be appreciated
^ well the thing is this, you might be trying too hard. sit down and jam them out and you might just come up with the perfect chorus/pre-chorus riffs. this is generally what i do, that and i always try to tape myself when i jam improv stuff so i can go back later and listen to it and pick out what i like. if you know some theory then ideally you can sit down and think of how you want the parts to sound and write it out.