The great thing about joining forums that are related to your niche is that your target audience is already there, so the traffic converts easily especially if you're giving great value.
To increase the sales you can use the forum marketplace (that's if the forum has one) to advertise your offer. You can also buy ad space on the forum like banners, some forums allow you to buy text links as well.
You can also check who's popular and highly active on the forum, then strike a good deal with them so that they can add your offer in their signature. Get someone who adds value, so that you can get a good return on investment.
Another great way is to buy some sticky posts, when your thread is at the top it will get a lot of traffic from people visiting that forum section. Choose an active forum section don't create a sticky post on a dead forum section.
You don't have to buy a sticky post on the marketplace only, you can also choose a highly active section and create a top quality article that offers a lot of value. If people like the article they'll surely click your signature.
But forums are different, just make sure you're not breaking any rules.
