Since Yoda is so old, I think the effects of his age should begin to appear in tRSB and RotR (Ep. 6). What I have in mind is that there should frequently be long, awkward pauses, followed by "I am wondering, why are you here?" or just the last bit of that sentence. He could also stop talking mid-sentence, Luke could prompt him or be confused, and then he could start talking to himself or something like Gollum/Smeagol.
Some example monologue for Yoda talking to himself could be...
YODA1: He is too old, yes, too old to begin the training.
YODA2: 800 years have I trained Jedi, too old I am not!
YODA1: A Jedi must have the deepest commitment, the most serious mind. My own council will I keep on who is to be trained!
YODA2: A serious mind have I! Hmm-hmm! (laughing)
YODA1: I cannot teach him, he has no patience.
YODA2: 800 years have I--
YODA1: Much anger in him, like his father.
YODA2: Hear you nothing I say?
He could have several debates with himself about whether or not to train himself, and it could go on and on...and more monologue:
YODA2: End your training now, choose the quick and easy path, as Vader did, become an agent of evil. Start down the dark path, forever it will dominate your destiny, consume you it will, as it did to Obi-Wan's apprentice.
YODA1: Much anger in him.
YODA2: Mudhole, slimy this is!
YODA1: My home this is!
It might be best to give Yoda a serious and a silly personality in tRSB and RotR to make it more clear to the audience that he is talking to himself. All of the examples I gave use available dialogue, and I only scratched the surface of what you could do with him.