Forums are hard. Most ad companies will not use them due to the large page views. You can try places like google ads, but they pay per click, not for views. The best thing you can do is sell on your own. Offer ads per month or something instead of x amount of views. Don't expect to earn much though. Its still tough, and forums are even harder.
You can try going to places like webhostingtalk.com or sitepointforums.com and seeing if anyone wants to buy advertising on your site. Or you can try contacting people on your own. I know the little advertise here thing under the banner gets one or two people a month asking about advertising.
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