Have the vendor answer questions to their satisfaction. Make sure that everyone's got his checklist with him, and marks off features as he sees them demonstrated to his satisfaction. It's often a good idea to have demos scheduled at your site, so you can involve many of the future users of the software without drastically disrupting your business. Remote demos utilizing modems and speakerphones are generally pretty simple to set up, and work quite well. They give you the advantage of getting multiple smaller demos, and the flexibility of working around the demands and time constraints of your critical personnel.
Software packages can look very different, and sometimes a 'pretty face' can be deceptive.
Make sure that you familiarize yourself with the software thoroughly enough to get a genuine feel for how it works. Don't be fooled by cosmetics or glitz. Make sure the vendor gives you the time in demos to see below the surface. Keep in mind that if you're selecting software to use in all the aspects of running your business, it's pretty unlikely that it's going to function to your exact needs. If you see features that aren't quite what you've got to have, talk it over with the vendor to find out how they envision changing the software.