How does one "group a bunch of revisions you've already committed to svn together" (see Jim T's answer) in Trac?
The very basic idea is simply to put a bug number into the revision log when you commit. If it's the first thing, then when you look down the logs you'll be able to see the bug numbers easily.
I know Redmine has an additional feature that checks for these bug numbers in the commit logs and attaches the revision to the bug, so you can see from the bug all the revisions that were involved in fixing it. I'd be suprised if trac doesn't have something similar.
TortoiseSVN has some support for ticketing systems that do this as well through the bugtraq properties. These integrate in different ways with the tracking software - from simple regex's that highlight the bug numbers and allow them to be displayed through to fully fledged modules that can be installed in tortoise to browse the open bugs whilst committing.