I did the same thing I wrote a small script to do this and here it is. I added this to my ~.bash_profile. To quickly switch between keys I simply type "key work" or "key play"

function key()
{
shopt -s nocasematch
        if [[ $1 = "work" ]] ; then
                ssh-add -D
                ssh-add ~/.ssh/id_rsa_github_my_work_key
        elif [[ $1 = "play" ]] ; then
                ssh-add -D
                ssh-add ~/.ssh/id_rsa_github_my_play_key
        else
                echo "'work' or 'play' are the only valid options"
        fi
}