I haven't out much time into my forum, but when I do I will give awards to my users, as I know it motivates people to be more activity and dedicated to something, I've seen this in on of the forums I am on.
Rewards will definitely attract more members to a forum. But what I am more interested in is to increase the referral commission. This way you can get more members to your forum and also run a contest for getting maximum number of active referrals. A member who got the maximum number of active referrals should be given reward.
But nowadays people start taking advantage of referral schemes and therefore referral commission to be released only if the referral makes a certain number of posts and stay active in the forum for a specified period of time.
In the earlier days when our forum was just a year old, we have an award system such as the most popular member or the most prolific. From what I remember there was a total of 5 awards and some consolation awards. The purpose is to inspire the community for more participation in the forum. But the awards were actually movie tickets that we send to the winners via postal mail. It was a great success because those who received their prizes were jubilant. By the way, the theme of our office forum is entertainment particularly movies and television.
Awards are essential to motivate the members for their active participation. It can be given as points, grades, etc., so that the members can foresee their next target and work towards that with active involvement in the forum activities.