This may not apply to you and as of yet it is all I can write, but I'll share it with you. First I choose some diminished chords in a common key. You will ALWAYS be able to come with catchy riffs using them in any order. Then at the bridge I use the help of a modulation chart to help me go into something dark like A minor. In this new key I write the solo based on the appropriate scales for A minor, mostly pentatonic and aeolian licks. I might just be a Randy Rhoads enthusiast!