In this case, one has not to add extra characters (since adding extra a's produces strings still in the language), but remove a nonempty substring of a's. Recall that the pumping lemma says that a sufficiently long string w can be written as w = xyz so that |y| >= 1 and xy^{i}z is in the language for all natural numbers i including 0.

If you need more hints, see here (PPT) or p. 5 here (PDF).