Involve the User Community Early, Including Your Detractors

"There's no way that if you get participation out of a person can they say you didn't rock it." ~Jam Master Jay
Many organizations feel that the best time to begin engaging the end-user community is during the pre-rollout testing phase of a project. While this is an important time to be talking with the user community, hopefully you are engaging them far earlier than this. Otherwise, you are at serious risk of adversely affecting their processes/workflows.
The best time to engage your user community is during the Envisioning portion of your project. This will allow you ample opportunity to:

• Identify Key Business processes involved/Develop templates or customizations
• Identify Business Issues/Find Workarounds
• Identify Processes requiring significant time or effort today/Improve existing process
• Identify Hard to Use Areas/Customize Training to address

When picking who to work with from the user community, don't slip into the trap of picking an "easy" audience comprised entirely of individuals who readily accept change. Pick some of your power-users who will likely be detractors. These folks are usually your biggest source of valuable information prior to rollout. This is because these folks are usually vocal with their opinions, so you might as well have them talking to you during development rather than to management during the rollout. This also has the side benefit of lowering your support overhead during the rollout phase as you usually have far more time to address their wants, concerns, and support needs during the development of the project than during its execution phase.
I know that while suggesting you involve your detractors early on in the project is an easy thing to do, it inevitably turns into a hard task to actually put into practice. It definitely can lead to lots of practice of skills you were not intending to hone during this particular project: patience, restraint… Well, you get the picture.
The fact of the matter is that while making your detractors a part of your team WILL add stress and work during the pre-execution phases of the project, it almost always leads to a better product in the end. Isn't that what everyone wants anyway?