Enter the desire to nudge.
Before you send off a note to soothe the soul, you need to do your research first. For example, an agent may state in their guidelines how long they take to respond. You might also find this information on their blog or via updates on twitter. Another place to check is on querytracker.com. Other writers will often post when they queried with dates on responses. Since most agents work in order, you should be able to get a picture of if your query has been read or not.
Some agents are quite clear on their nudge policy. For example, I include the dates of where I stand with queries and manuscripts right here on the blog. If you emailed me prior to those dates and haven't heard from me, I encourage you to reach out. Technology isn't perfect and I'm not either. There's always a chance your email is floating in no man's land.
To prove the point, I just received a (very polite) nudge last week and couldn't find the author's query anywhere. Seriously, me and one of our interns scoured the email boxes and were baffled. If this writer hadn't nudged, he would have been left wondering.
Once you've determined a nudge is appropriate, the next question is how to do it. The golden rule is to remember the definition of nudge.
Definition of NUDGE: To prod lightly; urge into actionNote the key word here is "lightly". Do not nudge with harsh words or a sense of entitlement. This is a fast way to get an agent's back up and put a sour taste in their mouth. At the same time, don't lay on the schmooze so thick we need waders. Polite, brief and to the point is the best approach.
I love the Mother.Write.Repeat. Nudge Advice from Krista VanDolzer. She has some very basic templates on her site that are perfect for following up with an agent or editor.
On a semi-related note, here's a sample email for asking an agent to consider a revised manuscript after you've already sent them a partial or a full. I've had this happen as well. Personally, I understand how getting fresh feedback can open your eyes to flaws in your manuscript. So long as I haven't read it yet I'm usually open to this, but every agent will be different. All you can do is ask.
In all your correspondence to industry professionals, the key is to be polite and keep your goals in sight.
Have you ever nudged? How did it go? Any tips?