I forked http://github.com/dpp/lift/tree/master to http://github.com/tjweir/lift/tree/master and I want to keep in sync with the update to dpp/lift. This is how I do it.

In my repo add a remote reference to dpp/lift:
$ git remote add dppmaster git://github.com/dpp/lift.git

Fetch
$ git fetch dppmaster

This will create a branch, so then you just have to merge back:
$ git checkout master
$ git merge dppmaster/master

Commit those new changes:
$ git commit -a -m "Sync to fork master"