Tony Byrne, of CMS Watch fame, just posted an excellent article on 5 common pitfalls encountered in a technology selection. People who have been through a number of these selections (from either the customer or vendor side) will recognize many of these issues.
I absolutely agree with Tony's point about becoming slaves to spreadsheets and the recommendation to consider more qualitative methods to examine the suitability of software (Pitfall #2). I also agree that the vendor that you work with is every bit as important as the feature/function/price of the product (Pitfall #4).
The most powerful technology selection technique you can leverage is talking to other users of the technology. Try to find other organizations that have similar business needs (not feature needs) and learn about their experiences with the technology and the support organization behind it. You will learn more this way than you will from hours of product demos and brochure reading.
Open source is not the only kind of software that has user communities. Commercial software has them too. Some vendors try to get in the way of customers (and prospects) learning from each other. Others encourage it. In fact, for some commercial software, customer forums are the most effective way of getting support. No matter what your software choice, use the community. If you can't/don't, you will suffer alone.