When I advertise I try to do two things.
The first thing is I try and find places to advertise that reflect my business. I may go through a handful of sites without gaining sales but that is how I go about trying to find a good advertising spot. Also, you need to figure out a goal for that ad. Are you trying to raise awareness where sales are not necessarily needed but appreciated or are you going for sales only. You wanna find a site that can give you not only give you good impressions but decent click ratio. (Click Ratio Definition
Secondly, I want to find a site that gives me decent sales. I want to find a site where I make my initial investment back and produce at least 2-5 more sales. This may seem like a low amount, but if I advertise every month on this particular site and I make back what I spent, plus additional sales then it is a spot worth keeping.
Advertising will take time to perfect. You need to be willing to spend money to make money. Spending $50 on advertising and making $0 and then saying to yourself "I can't afford to advertise anymore if I keep getting $0 back" will only hurt you. You need to find the right sites that will work for you. Also, focus on what you want...Brand awareness or sales? I try to do a combo of both. People will most likely remember your name so when the need arises it will be fresh in their mind and will most likely throw it out there in conversation.
Always research before advertising. You will waste money from time to time but if you do your homework you should make at least 1-2 sales that will hopefully return your investment on a particular site.