Hey Stephen, the gambling niche and to be more specific the football niche, are great niches to promote, if you know where to advertise and there's alot of money to be made, you have to think about the following factors,

1: think about the target audience, for instance here in the "UK" soccer's loved by alot of people young and old, so you would probably target soccer loving country's, age 18-60(again age varies with location).

2:look for soccer related or gambling related forums and blogs contact the owners about advertising, if they have a few hundred or couple thousand subscribers to their site that's perfect(never go with sites that seem dead or if they only have a small subscriber base, unless you know what your doing!) negotiate a fair price and place the order trust me when done right it can reap huge benefits, i recommend you try this with both banner ads, and solo-ads, and see how it goes, tweak and test and see what works best(lol I'm a poet and don't even know it), hope this helped :-) peace