There are so many ways...I have found that when I am just playing and goofing around on the guitar, great riffs will just come. Other times, I will pick up a different guitar I own that has a totally different sound or tuning and they will come that way. I also echo listening to as much music as you can, jam along to your favorite artists and just play. When you are having fun and not thinking about writing a riff in particular, the inspiration just hits in my experience! hope that helps