Hello, there! This is a very good question- postcards are an incredibly effective way to communicate with agents and casting directors, but they are only useful if you use them in the right way.
The most important thing to remember is that a postcard should be sent only when you have something new to share. Sometimes, in the effort to "stay in touch," an actor might send a postcard that simply states, "I just wanted to send you a little postcard to say hello." While it is a very nice message, most casting directors and agents would say that this type of postcard would be a waste of their time (and yours.)
So, when
should you send a postcard?
• As a "thank you" note to a casting director or agent after an audition (be specific about what you are thanking them for.)
• To update industry folks about a project you recently booked.
• To let people know about a special skill you've developed (for example, have you recently completed an intensive tap class and now consider yourself advanced? This is something that musical theater folks would want to know!)
• Alert the industry about a show you are in. List the specifics of the show, including full cast, as well as a way for the recipient to get comp tickets.
A few other hints:
• Ron is right- envelopes are not necessary. This is one place where you can save your money!
• I'd suggest only sending postcards to people you have met in person.
• Be sure to keep your address book updated. People change offices all the time, and you want to make sure everyone's contact information is correct before sending out postcards.
• Less is more- don't crowd the card with too much information.
• Write neatly- and if you don't have neat writing, type up the information and either print directly on the postcard, or print on a label to stick to the postcard later.
I hope this has been useful to you- best of luck with your future mailings!