Adventures in pushing back

First attempt: Try, but fail.
Partner 1 calls me: Do you have time to work on Project X?
Me: I'm swamped this week, but I can do it next week.
P1: Great, I'll tell Partner 2 you can do it.
P2: Glad you're on board. I scheduled a call with the client tomorrow. Get up to speed on the documents before then.

Second attempt: Fail.
Partner 3 calls me: Hi. I think you'll really like working with this new client. They need blah blah blah boring documents. Here is five minutes of background information. Great, so get started.
WHAT I SHOULD HAVE SAID: I won't be able to work on the documents this week. If you need them immediately, you should probably ask another associate.
WHAT I SAID: ...Okay. I won't be able to get started right away. What is the timeframe?
P3: We should get something to them soon.
Me: This will take a while, and we don't have all the information we need. What if I email the client today with a list of questions. That way they know we're on it.
P3: No, we need to send them all the documents.
Me: What if we send them a subset of documents. It will take me a while to do all of them.
P3: No, we need to send them all the documents.
WHAT I SHOULD HAVE SAID: I won't be able to complete the documents on this timeframe.
WHAT I SAID: Well, I'll take a look and see what I can do.
The next day I send him an email saying that I won't get the documents done until Friday.
Partner 3 calls me four more times over the next day and a half. BUT!

Third attempt: Silver lining.
P3: Just send me whatever you have. We need to get them something immediately.
Me: I haven't done anything. I don't have time to do this until Friday.
P3: Well, what can you get done today?
Me: Nothing. I can't do anything until Friday.
P3: I guess I will do them.
Me: Okay, thanks. I can take over on Friday.
Victory! I didn't cave or apologize. Did I piss off the partner, well, can't do much about that. Except stay up all night working. And I have done way too much of that lately.

My failure to clearly say NO resulted in the partner being annoyed, me being under a lot of pressure to spend quite a bit of time on work that I dislike, AND having to delegate a project that I really would like to do because I won't have time for it. Lesson learned... maybe this time it will sink in.