Elephant juice
When mouthed to someone, the lip-reading recipient will interpret this as "I love you". On their begging your pardon, you could then reply, "I said elephant juice, dummy."

This was briefly popular at our school until Steven Richardson blurted out "I love you!" to Andy Scott, missing the point entirely, but confirming what we already knew.
written by to*stie *e-*oast, approved by Ponky

On a similar note, mouthing "vacuum" looks a lot like you're saying "fuck you". Though this probably doesn't count because I just nicked it from an episode of Family Guy.
written by Ni*k Ke*t, approved by Ponky