An average person is exposed to over 3000 ads a day. This is shocking to most people because very few 
of those ads stick out and actually capture people’s attentions. More specifically, different people get interested in different ads.
This is why Google tracks people’s browsing history, and throws email ads that best fit what different people might be interested in, according to what people browse. This is why I’ve gotten emails asking if I was interested in getting a degree in fashion merchandising in a short term at a college, or if I was interested in getting coupons for clothing stores. Google already knew that I was interested in fashion. It’s kind of scary and intrusive, but it is advertisers’ job to get in front of the right target customers.
I would say the most basic but chief factor about advertising is to know who your target audience is. Who would want to buy your products/services? How do you want to grab the attention of your potential customers?
http://blogs.hbr.org/cs/2010/10/crowdsourcing_effective_advert.html
