Set objective of your contests. You can think of any contest that is able to engage customers and give them rewards. But unless you decide what your contest should do even after it ends, your strategy is doomed to fail.
For instance, offering something free may seem like a good idea as it can lure a large number of visitors. But how will you cope with the situation if most of these visitors disappear after having fun? So you need to define on paper (formally) what your contest is all about what it should achieve in the long run.
Set goals that are achievable
It should be noted that the goals that are hard to achieve usually scare the readers. So set goals that can be easily achieved. Easy goals will work as two-way street. It will not only determine the winner, but will also intensify the competition.
Create contests that assess the skills of your readers. You can conduct a brief questionnaire about your niche and industry. You can then offer free subscription to one of your featured services to the winner of that contest. Skills contest will vary business to business and industry to industry.
Design a challenging contest in order to engage the entrants, which should make them take the contest as a challenge.
Another thing that worried most of the website owners is that their customers get used to a contest that they once found attractive. This is something you should take as the red sign. If keep trying to lure customers with the same tactics all year, they might get bored and probably stop visiting your website after some time.