You can squash (join) commits with an Interactive Rebase. There is a pretty nice YouTube video which shows how to do this on the command line or with SmartGit:
If you are already a SmartGit user then you can select all your outgoing commits (by holding down the Ctrl key) and open the context menu (right click) to squash your commits.
It's very comfortable:
There is also a very nice tutorial from Atlassian which shows how it works: