If you're in the position of rebasing a branch down to a single commit, but many of those commits contain messages that are useful, then keep those messages in the final commit message.

When running an interactive rebase (git rebase -i my_branch~5), you have many options available to you. Fixup squashes and discards the commit message. I'll use this on commits I made that are literally just tags in a commit stream where I'm about to do a destructive action to my code (like during refactoring, you know the ones: 'git commit -m "update"') and I want to ensure I have a point to reset my branch to if I start screwing everything up. Squash just literally combines the commit with the next one, but preserves both commit messages. Use this for commits that are relevant to the named branch you're working on and are descriptive of what the branch was trying to accomplish.

Create a summary as the first sentence of the new commit message. It will show on any pretty-print log message output. Then literally combine all your commit messages into a paragraph (or more) for a detailed description. The information is still there, and it's still in the same place.