As with subjects any data which have been established can be deleted,
and any changes in already established data can be signalled using は:
明日パーティにいくか。
あした パーティ に いく か
"[Are you] going to the party tomorrow?"
うん、あなたは？
"Yeah, how about you?"
The information about "パーティに" and "いく" are unchanged, therefore
they are deleted. The subject changed to the original questioner, so the
change is signaled with は.
うん〜ん、いかない。
"No, I'm not going."
The information about "パーティに" is still deleted, but "いく"
comes back as "いかない" because it has changed form.
その後の映画は？
その あと の えいが は
"How about the movie afterwards?"
The established information "パーティ" changed to "映画" so we use は to
establish the change in understood information. Understood info that
hasn't changed is still left out, like the subject "僕" and the
action "いく".
This is pretty much how all anaphoric deletions work in Japanese whether
it be subjects, objects, or something else. In general, always attempt
to delete if possible. This section should also give you a feel for the
use of は in normal speech.