Christian Songwriting: Writer's Block - Riffs
First, have you tried taking two different riffs, and combining the first half of one with the second half of the other? The combination you come up with could, with minor adjustments, give you an entirely new riff.
Second, try taking a riff you're familiar with and reversing or inverting it. That's right, just play it backwards or upside down. (Bach used to invert music more than 200 years ago! For instance, his G Major French Suite.) Or try making some other changes. Remember though, you're not trying to steal someone else's riffs, you're merely attempting to get your own creative processes up and running. There's nothing wrong with obtaining your idea from someone else. Take a look at the obvious similarity between the bass riffs in Michael Jackson's "Billie Jean" (on the "Thriller" album) and the Billy Ocean/Keith Diamond song, "Caribbean Queen". Even the guitar licks are similar!
Billy Ocean's and Keith Diamond's idea may not necessarily have been inspired by "Billie Jean" - "Caribbean Queen" was copyrighted two years after Jackson's song - but the basic concepts are certainly alike.
Someone once said that originality is the art of concealing your sources. Make certain your sources are well concealed. Nobody wants to be sued for plagiarism.
Now read: Writer's Block - Style Change