Keeping in touch is important. In other industries , if you go on an interview, it is customary to send an email to follow up and thank your possible future employer for the opportunity.
Why should the casting process be any different? It's not about whether it works or not.
If you weren't meant for the part, you're still not going to get it, but the chances are the next time you come across the same casting director, they might remember you and it could mean that one extra vote to get you cast.
Not only that, it's just plain human decency to say thank you, taking that one extra step is not going to hurt anyone.
Okay, so here's what I'm talking about. I'm not saying email - many times CDs won't tell you their email address anyway (for obvious reasons), but if you're at their office, you already know the mailing address.
Carry the "Thank You" postcards within your Swagfolio on you at all times. If you're a premium member and haven't received your Swagfolio package, claim it here. As you wait for your audition, take out a pen and address the post card.
On your way out, write a few words on it, the more recognizable and custom you can make it the better. If the CD mentioned something about your performance, reference it. Even if it all went badly - no matter. Put a few words on there, slap a stamp on it, and drop it in the nearest mailbox.
It's not hard. The thought is what matters. Your name should be at the bottom of the note. When the CD tries to remember who you are, they will google you. Luckily, you already have a great website with IADB so they will find you easily.
They'll visit your website, they might check your credits, who knows they might see something they haven't noticed before.
Worst case scenario... you just wasted a postcard and a stamp. Best case scenario... the sky's the limit.