Your company has a perfectly good wiki but your (otherwise intelligent) co-workers insist on emailing you Microsoft Word documents to review. Your gentle guidance has been ignored. Your snarky comments have been equally ineffective. What do you do? I suggest the following method. Try it (at your own risk) and let me know how it works.
A co-worker has sent you and four or five other colleagues a MS Word document that summarizes some research that he did. The document contains some simple headings and paragraphs. You have a lot of ideas and feedback to give but, rather than give it all at once, you trickle it out and have your co-worker feel the pain of merging your edits.
Draft 1: Re-Format. Don't really change anything, just format the heck out of the document. Change the styles around. Send it back with a cryptic filename like [irrelevant name]_[your middle name]_final.doc. Mail it with a note saying that it looks good and you had some slight wording changes. Leave it up to your co-worker to figure out that nothing has really changed.
Draft 2: Re-Organize. A day after the first draft, take the original document you received and re-organize it. Re-order the sections and a few sentences here and there. Make sure that you turn on "track changes" so that the whole document is a multi-hued mosaic of change notifications. Name it [original filename]_new.doc. Send it with a note saying that you discussed it with another colleague and got some new ideas but "it is really coming along!"
Draft 3: Edit. This is the version where you provide your real feedback. Start with your Draft 1. Make your edits. Name it [original filename].doc. Send it with a note saying how excited you are about this project and have been "thinking about it non-stop." Schedule a meeting to go over it and ask that your colleague send out a merged final draft for everyone's review. During the meeting, walk through everyone's feedback. By this time, your colleague is probably totally fried and is ready for a new way of working. At the end of the meeting, innocently say something like "you know, this might have been easier if we had worked in the wiki."
This method works even better if you have a co-conspiritor working with you doing the same thing. During the process, watch out for signs of mental instability or fragility and remove all sharp objects from the office. Make sure that the Employee Assistance Program posters are visible and well placed.
All humor aside, sometimes the only way to change people's ingrained behavior is to offer an alternative that is substantially easier. It has to be a big improvement because old habits are hard to break. Starting to use a wiki can be hard for people accustomed to Microsoft Word. They will initially get frustrated and resort to what they know. If you can't eliminate the learning curve of a wiki, you can expose the inefficiency of collaborating without one. The next time they launch Word, they will remember their painful experience and think twice.