CMSWatch: Featured Opinion: 5 Biggest Mistakes in CMS Selection provides some excellent pointers on pitfalls to avoid when selecting a CMS.
While they are all good points, the most fundamental recommendation, imho, is to create a formal, diverse, selection team. At a minimum, according to the article, it should include both IT and content development staff. This seems obvious but I have heard of many instances where IT simply handed down a solution to the content folks without their involvement in the evaluation of products. Less frequent are cases where the content staff buy software without consulting IT and then ask IT to install it. IT people LOVE that. 🙂
The content staff are the primary users and are the best judges of authoring and publishing functionality. IT staff can best judge the infrastructural requirements and performance measures as well as the depth of their knowledge in the technology on which the candidate systems are based. You must have both of them well represented in the selection process.
(Thanks to Column Two for the pointer.)