How should I do marketing to receive more replies from potential customers?
If you have a more robust budget, test out targeted online ads on sites that your prospects visit and do a Google Adwords campaign. That said, cold calling and direct mail are still useful marketing tools if they're used correctly. You should analyze your approach, and the content of your message in each medium, as well as how your brochure looks.
You have a much better chance of getting results with a professionally designed and written piece, using a professional cold calling firm to engage your prospects. In many cases, professional firms using a carefully developed script can get much better returns for you. Check out It's Your Call and MarketReach.
Best of luck with growing your business!