Don't go to the U.S. Sentencing Commission's website for help, at least until it's up and running again after Anonymous hacked it-twice. While it was still operating, the Commission's series of guideline primers, such as this Drug Primer, was a useful starting point for learning the basics of sentencing guideline issues.
More helpful for defense attorneys is the Sentencing Resources page at fd.org, including its excellent deconstructing the guidelines series.
Also don't miss E.D.N.Y. District Judge John Gleeson's "Memorandum Explaining a Policy Disagreement with the Drug Trafficking Guideline" in U.S. v. Diaz and N.D. Iowa District Judge Mark W. Bennett's opinion deconstructing the career offender guideline in U.S. v. Newhouse. Prof. Berman's Sentencing Law & Policy Blog has summaries, Diaz here and Newhouse here, calling the opinions "instant classics."