Prompted by a discussion on another site, I was going to suggest this myself, but found this existing feature request instead.
Specifically, I feel that the problem with the current system is that new users, who may not realize that there's anything wrong with making dozens of tiny edits to their post a few minutes apart, will not see any warning that what they're doing is in any way discouraged before their post suddenly turns CW.
What I would suggest is that, if a post has three or more revisions created by the same user within the previous 24 hours (specific thresholds subject to adjustment, of course), the next time that user tries to edit it, the edit form should display something like the following warning:
You have already edited this question/answer several times recently.
Please try to avoid making too many minor edits, as this will needlessly bump the question to the front page. Instead, use the preview pane and save up multiple small changes into a single substantial edit that addresses all issues that you feel should be fixed.
If you do notice a mistake after saving an edit, try to fix it within 5 minutes, as doing so will not count as an additional edit. See the help center for more information on editing.
Ps. The proposed warning message above doesn't actually even mention Community Wiki in any way. Nonetheless, simply by informing new users that continual editing of posts is not recommended here, I believe it would significantly decrease the incidence — and, more importantly, the surprise factor — of unexpected CW-fication.
I do feel that it might not be a bad idea to also show a separate warning just before a post is about to turn CW, something like:
You have already edited this question/answer <n> times. Further editing will automatically mark it as Community Wiki. For more information, see What are "Community Wiki" posts?
(For that matter, I'd also like to see the CW-ification threshold changed so that sufficiently old edits would not count against it, or would count only partially. This would reduce the risk of CW-ification of old popular answers over time simply due to to occasional infrequent updates or corrections. But that's really something for a separate request...)