How do I write riffs for my guitar?

Question:

How do I write riffs for my guitar?

I've been playing for a few years, and recently I've been throwing chords together, some good, some not so good. I just need to know whats the best way to write a riff based of a few chords. example
I got :Am, to a G, to a C, to an E7. What would be a good riff? Any free sites with tips would be very appriciated.

Best Answer - Chosen by Asker: To start out, learn the minor pentatonic scales for the chords you are using. Many melodies and riffs are found in these scales. After you can play the pentatonic scale, strum a few bars of the chord (say A minor) then play a riff in the Am pentatonic scale. Then go to your G chord. Fool around with the riff until the ending note sounds good leading into a G chord. Now do the same thing with the G chord and Gm pentatonic scale. You will start finding riffs all over the place if you use these scales and related blues scales.
A great book is "Riffs" by Rikky Rooksby. This is an excellent author who writes outstanding books on composing on guitar and keyboards.
Good luck.