Check out this awesome series by author Jonathan Auxier for an example of this (especially this one on surviving no-shows at a book event). It doesn't have to be this in-depth or focused at writers, either; think of ways you can help your readers, too (what would be interesting to THEM?) or help a fellow author with promotion!
Pay-it-forward marketing drives a heck of a lot more traffic and interest than constant selfie marketing.
When my clients sign up for editor critiques or pitches at conferences
This really could vary by agent, but personally, I advise my clients not to sign up for these (read: CLIENTS. Not speaking here to unagented authors). I can submit to editors any time - and honestly, it's tough to beat a first impression. Yes, sometimes an editor will be intrigued by opening pages of a manuscript, but that's my job when I pitch - to get an editor salivating to read! Particularly with a critique, an editor is going in with the express intention of finding something to critique. Why add that negative impression as the first?
I would SO much rather send a polished version for consideration, not just because of first impressions, but also because it's also possible that something an editor has already seen, even if they're interested, will sit longer on the desk for consideration. They know what to expect; the element of mystery and surprise is gone.
It also puts me in an awkward situation of having to send to that editor to consider if he/she said "sure, send it to me!", even if I think a different editor at that house would be a better fit.
There's plenty an agented or published author can do at a conference besides pitch - learn, network, teach - far more valuable and relevant to that stage of his/her career!