Target
You must get your products or services in front of a targeted market. Just throwing up ads haphazardly and hoping you get traffic and sales is like trying to hit a bulls eye with a load of buckshot. Sure you might hit a couple but the rest is just scattered out aimlessly. You need to know your target market. Study who they are. Are they rich or poor? Male or female? Know what makes them tick. Create a profile of who your target market is and then advertise to them.
Traffic
So now you know who to target, the next thing to do is to get traffic. Obviously getting a lot of traffic to your site increases your chances of sales. Study up on ways to increase traffic. Implement the easiest and less expensive ones first. Once traffic increases and sales start coming in, you can use profits to do more paid advertising to your target market.
Conversion
You've got the traffic coming in, now you have to convert them to buy. This can be the most difficult. Make sure you have a good sales page. If you have a high amount of traffic but no sales then something is wrong with your sales copy and you have to get it changed ASAP. Hiring a good copywriter can be expensive but it can make all the difference in the world. People don't want to be sold, they want to be convinced. If you can't afford a good copywriter, study copywriting on your own and know how to set up a good sales page.
Backend
There is where you get repeat sales. Make sure you capture your customers email address and stay in contact with them. Send them emails with useful tips and information relating to your products. In these emails, include offers for more of your products and services. If the customer was happy with their first purchase from you they will be more likely to buy from you again. Repeat customers are worth their weight in gold. Keep them happy and you have a steady stream of income.