Amy Einsohn's The Copyeditor's Handbook is a terrific resource if you want to refresh your skills for a copy editing job. The book is full of exercises with answer keys.
I worked through the book in 2000 while I was freelancing. The exercises were a good reminder of how important it is to slow down when you're editing. (I'm on a newspaper copy desk now, and slow is not the order of the day. We sometimes slam through stories in a few minutes, which is stressful on a number of levels.) But if you have the luxury of not working on an immediate deadline, take your time.