Obi-Wan died in ANH, but came back to Luke in spirit form in ESB and ROTJ. So I would guess that Yoda could come back in spirit form to train Luke. Heck, for that matter, Obi-Wan, Anakin and Yoda could all train him since we see all 3 in spirit form at the end of ROTJ.Because Yoda died?
There are also the Jedi holocrons out there that Luke might be able to find and learn from.