The first one asked whether or not you should send a "thank you" email to an agent after getting a response to your query.
Truly, there is no need to send a thank you note. You don't have to thank me for reading your query. I invited you to send it (as per my guidelines), and I reviewed it and made a decision whether or not I'm interested in seeing your work. That's it. Not only that, but a thank you note just becomes another email in an already crowded inbox. When I receive a thank you email, as soon as I realize what it is, I delete it, even if that means that I don't finish reading it. It's not meant to be rude at all, it's just a time issue. My mother raised me to always thank someone when they've done something for me, so believe me when I say that I understand how it goes against your grain NOT to send a thank you note. But hold back. Just don't do it.
The other question was in a few parts, so I'm going to post it directly here:
The only contradictory advice I've heard deals with what an agent's response means. I understand rejection form letters---they mean move on. If I'm lucky enough to get any suggestions or comments, I strongly consider using them. But I've heard that silence can mean a lot of different things. I've heard it can mean 'no' or 'I'm extremely busy right now and I'll get to it'. So if you have time, maybe you can shed some light on this for me. Do I call or not call? Do I resubmit or not resubmit?
First things first--you never, ever call. If you're going to follow up at all, you follow up via email. As for when do you follow up, first check what the agent's guidelines state. On our website, we state that no one should follow up on a partial or full manuscript prior to eight weeks (unless they've been offered representation). After eight weeks, if you'd like to follow up, make sure to do it within the same correspondence so the work is right there for me to connect you to it, and keep it concise. An agent's website also might state that they'll contact you if they're interested, but otherwise they don't respond. If that's the case, the silence can mean NO.
But in truth, silence really can mean MANY things. So I can't really shed more light on it for you than that. I hope what advice I gave did help though!