Hey. So Often Times I'll Be Sitting Here And Come Up With A Really Sweet Riff. I'll Play The Riff A Couple Times Over And Think Of Sweet Drums And Bass For The Riff And Then I Just Get Stuck. I Guess You Could Call It Writer's Block. But Anyways, How Do All You Songwriters Out There Make A Riff Into A Whole Song?
I record all my idea's.
After recording them I save them and start browsing for previous recording which might go well with the new riff.

If I don't find any, I just let it be, and return to it the next day or something.

If I do find something that will go well with it, or might go well, I use the recording to play the riff again, but in a different tempo / whatever. I record the new riff, add it to the old, and start adding a drumtrack and bassline.
Then I start working on how to make the riff connect to the previous. Something like a bridge, breakdown, drummfill, whatever will do.

Best thing about this way of working is that even if I can;t use the idea right away, I am building a big archive of good idea's that can help me later on.

And what works well too is keeping your new riff in mind and just going through the daily grind, the best idea's I've had so far struck me on my daily commute to work, while sitting behind my desk, etc.
Would all the posters here please read the rules, this should be in the techniques forum.

I would move it but it's probably best if you repost it there minus the capitalisation.

Filth, pure filth... That's what you are.