1. Provide extra subjects to your web site. Most free ad sites only allow you to submit your web site to one category or label. This'll allow you to submit it to many.
2. Step-up the sensed value of your product by making your offer rare. You could use limited time bonuses, low prices, low quantities, etc.
3. Use SWOT analysis and use it as your "Unique Selling Proposition". It's the reason why people buy your products and get more orders.
4. Sell your products or services to a unique niche market. For example, instead of selling your mountain bike book to all bikers, target it toward fly mountain bikers.