Hey there,
Whether you use postcards or a letter, the important thing is to make sure you're grabbing the reader's attention in a way that makes him/her want to contact you to learn more. One idea that's been proven to work is to use a physical attention grabber. In the case of a professional service business like virtual assisting, you could use a glue dot to attach a nice, shiny penny to the top of a letter and then, in the opening sentence of the letter, say something about how you included that penny to show the person that you can help put money in their pocket, save them money, etc. Then launch into a discussion about all of the benefits that working with you will bring into their life. And not just the generic "save time"...tell him how much time your services can save him each week. Ten hours? Twenty hours? Specifics are important, as they can help paint an appealing picture in your prospect's mind. You want her to imagine what she would do with an extra twenty hours each week...most busy business owners (as well all know all too well) would salivate at the thought of twenty extra hours each week!
Before you spend any cash on your direct mail campaign, you might want to do a little research on what works with direct marketing. There's a book by Bob Stone called
Successful Direct Marketing Methods and another by John Caples called
Tested Advertising Methods that will give you some great pointers. (I found copies of both at my local library, so you can start there if you don't want to buy your own copies.)
If & when you do send your campaign, come back and let us know how it did!