Combining multiple commits before pushing in Git

What you want to do is referred to as "squashing" in git. There are lots of options when you're doing this (too many?) but if you just want to merge all of your unpushed commits into a single commit, do this:

git rebase -i origin/master

This will bring up your text editor (-i is for "interactive") with a file that looks like this:

pick 16b5fcc Code in, tests not passing
pick c964dea Getting closer
pick 06cf8ee Something changed
pick 396b4a3 Tests pass
pick 9be7fdb Better comments
pick 7dba9cb All done

Change all the pick to squash (or s) except the first one:

pick 16b5fcc Code in, tests not passing
squash c964dea Getting closer
squash 06cf8ee Something changed
squash 396b4a3 Tests pass
squash 9be7fdb Better comments
squash 7dba9cb All done

Save your file and exit your editor. Then another text editor will open to let you combine the commit messages from all of the commits into one big commit message.

Voila! Googling "git squashing" will give you explanations of all the other options available.

If you have lots of commits and you only want to squash the last X commits, find the commit ID of the commit from which you want to start squashing and do

git rebase -i <that_commit_id>

Then proceed as described in leopd's answer, changing all the picks to squashes except the first one.

Example:

871adf OK, feature Z is fully implemented      --- newer commit --┐
0c3317 Whoops, not yet...                                         |
87871a I'm ready!                                                 |
643d0e Code cleanup                                               |-- Join these into one
afb581 Fix this and that                                          |
4e9baa Cool implementation                                        |
d94e78 Prepare the workbench for feature Z     -------------------┘
6394dc Feature Y                               --- older commit

You can either do this (write the number of commits):

git rebase --interactive HEAD~[7]

Or this (write the hash of the last commit you don't want to squash):

git rebase --interactive 6394dc