there was a good thread on this not long ago in this forum
entitled "Creational Slowdown", but i did a search and couldnt find it..
anyway i think i said some of the following:
1. Listen to lots of different music, completely different to what you are normally listening to... can give you plenty of new ideas
2. Maybe its a matter of putting your ideas towards some other musical area... instead of worrying about good riffs, maybe just fuck the riffs and play a bunch of shitty powerchords and put your efforts into song structure.. Maybe instead of writing an epic 10 minute masterpiece its time to write a 3 minute verse-chorus-verse song?
3. Try a completely new approach.... maybe write a story and try and put music to it afterwards?
Or draw some pictures and try and convert those images into music..?
Use your imagination
4. Just play.. dont worry about trying to write a good riff, just improvise for hours on end while your doing whatever.
5. Change things like tunings, try dropped D.... put a capo on.. whatever... or tell yourself "im gonna write a riff using only the low E and D string" and see what happens...
6 and beyond. Theres heaps more ideas along the same lines, the main idea is just to change things around...
but yeh time will fix it, dont try and force yourself to write particular riffs coz it rarely works..