I don't mean at the query stage. Too many agents these days have adopted "no response means no" to do anything but assume lack of interest if they don't reply. (They are idiots, but that's ok, more good writers for ME!)
I mean at the requested full, or ongoing conversation stage. In other words, once I've expressed interest in your work. At that point, DO NOT ASSUME. Even when you fear the worst, never send an email that says "I guess you're not interested so I'm going to self-publish" because that makes it very easy for me to say "oh, ok, see ya" and move your file on my submission log to the trash.
If you are absolutely certain that I'm a complete dunderhead who has failed to see the brilliance of your work instantly here are the two remedies available to the Smart Writer:
1. Email for a status update. That email looks like this:
I queried you on Jan 10, 1037 for my novel Apocalypse Poelle. You asked for the full on Jan 11, 1037. I wanted to see if you are still considering the novel. Thank you for your time and consideration.
No Fingernails or Hair Left Writer
Hopefully you will receive a reply that starts "oh my gosh, I'm so sorry to be so tardy in getting back to you" and goes on to either ask for more time or let you know what the reading results are.
In the unfortunate instance when Snookums fails to reply, you ping once a month for three months. Then, and only then, do you avail yourself of the second option:
(2) Withdraw the submission. That email looks like this:
I queried you on Jan 10, 1037 for my novel Apocalypse Poelle. You asked for the full on Jan 11, 1037. I am withdrawing my novel from consideration. Thank you for your time.
(not so )Lovingly yours,
What you don't EVER EVER EVER say is "I assume you're not interested."
Remember, as a writer, you are PITCHING your work. That means you approach this from the seller's perspective. Never assume no.
Honestly if I assumed "no" every time I haven't heard from an editor in what I consider a timely fashion (ie instantly) I'd have sold about four books total.
It's very easy to shoot yourself in the foot. Aim higher.