You should sign with a manager when the opportunities a manager can create are better than the ones you can create yourself. Or if you are spending more time deal making than writing songs, you should get a manager. A good manager drums up new business, coordinates your existing business activities, and generally advises you and propels your career forward. Managers get a percentage of your earnings, so your ideal candidate (a) believes in your career and its possibilities, (b) is aggressive in seeking out new revenue streams, and (c) is looking out for your long-term best interests in any deals that are made.