I know you might not want to hear this, but 100 isn't many at all. Like Michelle said, it's a numbers game. In addition to direct mail marketing you might also want to expand your internet marketing. Just building a website is not enough. Since you have one it's a good idea to take things to the next level. While I can appreciate those on a shoestring business, just like any business.. it does take money to make money. Sure you can keep your costs down but in exchange you need to expect to spend MORE time marketing and right there time=money.
If you are going to do postcards, keep them simple but offer something of relevance, something that is going to make them call you or even just visit your website. Offer a free article or newsletter off your website and use the postcards to advertise your website and you kill two birds with one stone. Your goal is to get them to call you first.. if not get them to your website and get them to browse the site..your website will be an extension of your direct mail marketing so make it count!
Look at your website through your customer's eyes, not your own, you'll be surprised that what appeals to you might not appeal to your target audience. Make sure your website is search engine friendly, optimized, and that you have good SEO and submission services, change content on a month by month basis, websites with outdated information will get pushed down lower to accommodate sites that do update.