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Go to Bitbucket and create a new repository (its better to have an empty repo)
    git clone git@bitbucket.org:abc/myforkedrepo.git
    cd myforkedrepo

Now add Github repo as a new remote in Bitbucket called "sync"
    git remote add sync git@github.com:def/originalrepo.git

Verify what are the remotes currently being setup for "myforkedrepo". This following command should show "fetch" and "push" for two remotes i.e. "origin" and "sync"
    git remote -v

Now do a pull from the "master" branch in the "sync" remote 
    git pull sync master

Setup a local branch called "github"track the "sync" remote's "master" branch
    git branch --set-upstream github sync/master

Now push the local "master" branch to the "origin" remote in Bitbucket.
    git push -u origin master
    

Courtesy: http://stackoverflow.com/questions/8137997/forking-from-github-to-bitbucket