So, you want to push something but you are getting a non-fast forward error.
Here's how to update your local repository so your push will be a fast
forward.

If you type git status you should see something like this.

# On branch master
# Your branch and 'origin/master' have diverged,
# and have 2 and 2 different commit(s) each, respectively.
#
nothing to commit (working directory clean)

This means that the history of the repository looks something like this

         C---D   master
        /
   A---B---E---F origin/master

Where C and D are two commits you made, and E and F are two commits someone
else made and pushed to the up stream repository.

There are two ways to fix this. One you could do a merge which results in this

         C---D----
        /         \
   A---B---E---F---M master, origin/master

Where M is a merge commit of master and origin/master, and when you push the
history will literally have this small loop in it. We will _NOT_ be doing this.

Options two is to do a rebase which results in this

                 C'--D'  master
                /
   A---B---E---F origin/master

Where C' and D' are the same commits as before, with the exception of having to
handle possible collisions.

This is what we'll be doing. To perform this operation simply type

> git rebase origin/master master

or if you are on the branch master, you can simply use

> git rebase origin/master

If there are no collisions, your done. Here's some example output.

First, rewinding head to replay your work on top of it...
Applying: commit C's message
Applying: commit D's message

If there are collisions, you have to resolve them. Once resolved you

> git add file
> git rebase --continue

and it continues rebaseing your changes. If you want to abort the rebase
because the resolve was too nasty you can

> git rebase --abort

which will bring you back to

         C---D   master
        /
   A---B---E---F origin/master

once your done rebaseing you can simply do a git push and it should work out
now. Assuming of course that someone else didn't push more stuff ...

Enjoy,
Patrik