In this op-ed a couple professors suggest changing the federal system to one which mirrors Virginia's sentencing guidelines.
In Virginia guidelines are entirely optional and judges can depart from them either up or down at whim. The judge must give a reason but the reasons given are often rather weak (usually because the same reason was already accounted for in the recommendation) and a departure from the voluntary guidelines cannot be appealed.
All that said, it is amazing how many of the judges actually stick within the guidelines. Of course, judges in Virginia are creatures of the Legislature, serving at its periodic whim. If the Legislature feels they have been going too far afield the next time they come up for election they might have to find a new job. This has some good effects in that judges will tend to follow legislatively mandated guidelines. It also has some bad effects as anyone who has watched judges looking over their shoulder whenever they are asked to rule on an issue which is hot with the Legislature (e.g. DUI interpretations and sentencings).
I'm not sure what incentive would cause federal judges to follow voluntary guidelines.
Lv Sentencing Law & Policy