Do you have more than one account for GitHub, i.e. one for professional work dev and another for personal side projects?

If you're using SSH to clone repositories this might help...

Setting up your SSH keys

I won't copy the existing GitHub docs for generating keys

Once you've created the keys you should add them to your GitHub accounts, again their article would be better than mine...

Now that you've got at least 2 SSH keys you need to add some config to ~/.ssh/config, if that file doesn't exist yet you can create it.

In that file, you need to add an entry like this for each GitHub account

Host github.com
    Hostname github.com
    User git
    AddKeysToAgent yes
    IdentitiesOnly yes
    IdentityFile ~/.ssh/id_fizz

For each one you need to change the Host and the IdentityFile. i.e. if you had a personal and a work account you could set it up like this:

Host github.com
    Hostname github.com
    User git
    AddKeysToAgent yes
    IdentitiesOnly yes
    IdentityFile ~/.ssh/id_fizz

Host work-github.com
    Hostname github.com
    User git
    AddKeysToAgent yes
    IdentitiesOnly yes
    IdentityFile ~/.ssh/id_buzz

Now you've done that you should be able to be able to test you're connections like so:

$ ssh -T git@github.com
Hi <Personal GitHub Username>! You've successfully authenticated, but GitHub does not provide shell access.

$ ssh -T git@work-github.com
Hi <Work GitHub Username>! You've successfully authenticated, but GitHub does not provide shell access.

Now you've done this you can clone repos like this:

### personal ###
$ git clone git@github.com:fizz/buzz.git


### work ###
$ git clone git@work-github.com:fizz/buzz.git

Updating your Git config to manage this for you

You might want to set up you're gitconfig files to handle this for you. There are a few ways of doing this but I do it like this.
In my root .gitconfig file, I have this configured:

[includeIf "gitdir:~/work/"]
    path = ~/work/.gitconfig-work

[includeIf "gitdir:~/personal/"]
    path = ~/personal/.gitconfig-personal

And then in my ~/personal/ and ~/work/ directory, I have gitconfig that is specific to each user. For example:

.gitconfig-personal

[user]
  email = buzz.fizz@personal.co
  name = mg
  username = buzz

.gitconfig-work

[user]
  email = fizz.buzz@work.co
  name = mg
  username = fizz

[url "git@work-github.com:"]
  insteadOf = "git@github.com:"

In the work based config I have a URL remapping.
So in you're work directory you no longer need to do $ git clone git@work-github.com:fizz/buzz.git you can do $ git clone git@github.com:fizz/buzz.git as you would when you normally clone something from GitHub.

(Thanks to @ccoveille for making me aware of this!)

I recently found out how to do it and thought I'd share, there might be better ways of doing it. Please let me know if there is!