git push --tags is a separate operation to git push since pushing tags should be a conscious choice to avoid accidentally pushing the wrong one. That's fine. But how do I push them both simultaneously / atomically? (git push && git push --tags is not perfectly atomic.)

Update August 2020

As mentioned originally in this answer by SoBeRich, and in my own answer, as of git 2.4.x

git push --atomic origin <branch name> <tag>

(Note: this actually work with HTTPS only with Git 2.24)

Update May 2015

As of git 2.4.1, you can do

git config --global push.followTags true

If set to true enable --follow-tags option by default.
You may override this configuration at time of push by specifying --no-follow-tags.

As noted in this thread by Matt Rogers answering Wes Hurd:

--follow-tags only pushes annotated tags.

git tag -a -m "I'm an annotation" <tagname>

That would be pushed (as opposed to git tag <tagname>, a lightweight tag, which would not be pushed, as I mentioned here)

Update April 2013

Since git 1.8.3 (April 22d, 2013), you no longer have to do 2 commands to push branches, and then to push tags:

The new "--follow-tags" option tells "git push" to push relevant annotated tags when pushing branches out.

You can now try, when pushing new commits:

git push --follow-tags

That won't push all the local tags though, only the one referenced by commits which are pushed with the git push.

Git 2.4.1+ (Q2 2015) will introduce the option push.followTags: see "How to make “git push” include tags within a branch?".

Original answer, September 2010

The nuclear option would be git push --mirror, which will push all refs under refs/.

You can also push just one tag with your current branch commit:

git push origin : v1.0.0 

You can combine the --tags option with a refspec like:

git push origin --tags :

(since --tags means: All refs under refs/tags are pushed, in addition to refspecs explicitly listed on the command line)

You also have this entry "Pushing branches and tags with a single "git push" invocation"

A handy tip was just posted to the Git mailing list by Zoltán Füzesi:

I use .git/config to solve this:

[remote "origin"]
    url = ...
    fetch = +refs/heads/*:refs/remotes/origin/*
    push = +refs/heads/*
    push = +refs/tags/*

With these lines added git push origin will upload all your branches and tags. If you want to upload only some of them, you can enumerate them.

Haven't tried it myself yet, but it looks like it might be useful until some other way of pushing branches and tags at the same time is added to git push.
On the other hand, I don't mind typing:

$ git push && git push --tags

Beware, as commented by Aseem Kishore

push = +refs/heads/* will force-pushes all your branches.

This bit me just now, so FYI.

René Scheibe adds this interesting comment:

The --follow-tags parameter is misleading as only tags under .git/refs/tags are considered.
If git gc is run, tags are moved from .git/refs/tags to .git/packed-refs. Afterwards git push --follow-tags ... does not work as expected anymore.