Question: staying in touch with an agent from a conference who liked your work

In March 2014 I attended a pitch session organized with SCBWI and met Agent X. Already in the introduction when she listened to everyone’s ideas, she expressed that she was interested in my project.  Then after my 5 minute pitch and her reading a sample query that I addressed to her as if it were by mail, she was explicit in her interest.  

During the ninety minutes she told me three times that she was interested in my manuscript, to  send it when it when ready.  She gave me her business card and told me she was very pleased to meet me. I told her I would send her something by Christmas.

I will not have a polished manuscript by Christmas but revising first draft.

My question to you is: Should I contact her (end of November)and say that I am still working on the manuscript and need a few more months to send her my query and that she will be the first.  Or should I just wait until I am ready.  This may be June 2015.

It's never wrong to stay in touch (gently) with someone who has expressed interest in your work.  By gently I mean, you recognize that while your manuscript and her interest in it are of Burning Hot Importance to you, they fall somewhat to the cooler side of the lava flow for the agent.  Yes, she's interested but she's not planning her vacation schedule around your promised delivery date.

Here's how you do this:  About 30 days before the date you'd thought the manuscript would be ready (in your case December, so November) you drop her a SHORT email reminding her that you met at the conference, she expressed interest in your work, and while you thought you'd be sending in December, you are still in a preliminary draft.

This email is NO LONGER than five sentences of no more than 15 words each.  In other words: short! In other words: you're going to have several drafts of this, and it's going to take longer than five minutes to write. It's hard to write succinctly, but it is imperative you do so. (Your question to me above is 178 words in four paragraphs. Your email to her will be one paragraph of no more than 75 words.)

Why is it imperative to be short?  Because you want to convey the impression that you write well (long emails conveying ONE piece of information do NOT do that) and you only want her to use three seconds to get the info.

If you aren't ready six months later, repeat the above.

Don't fret if you don't hear back on something like this. Don't fret that she's dead, fled, or no longer interested.  Time to worry about those things when the manuscript is ready.