Modifying a wrong commit message in a svn/trac combo

Sometimes I screw up a commit comment in subversion, and need to fix it. I can never remember how to do so. 'svnadmin setlog' seems obviously the way to go, but when I try to use that command I get an error message:

svnadmin: Repository has not been enabled to accept revision propchanges;
ask the administrator to create a pre-revprop-change hook

Here's the right way to do it, including extra steps which make Trac aware of the change:
modifying a wrong commit message in a svn/trac combo

The need for the trac-sync step implies that trac uses a subversion commit hook to record the details of each subversion commit.