Help Hiring A Contractor

We have been very fortunate when hiring contractors in the past, mainly due to our in-depth screening. You can never spend too much time asking questions and researching the credentials of a contractor. With our glowing past of hiring great contractors we were asked to help our sister-in-law interview and hire a contractor for her kitchen renovation. We tried at first using a contractor we have used in the past however he was unavailable for months and my sister couldn't wait. We ended up interview and researching several contractors and thought we found the perfect crew. They were referred to us by another family member...boy were we wrong! Constant time delays, inadequate workmanship and constant excuses. What did we do wrong? We checked all credentials, references and more but still got screwed! We have found a lot of great tools and tips here on this site, which will be used for future project but we want to know from YOU...what else can be done to ensure you're getting a great contractor? My husband and I are reno junkies so any help you can provide would be greatly appreciated!