Changes in ad position
Recently, I removed the two large adverts (300x250) at the top of the forum to improve the usability for non-logged in members. Instead, I replaced them with one thinner advert (728x90) along the top. This allows more content above the fold and makes the ads slightly less in your face.
Normally, on the third post of a thread, you get a 728x90 ad. This has now been replaced with a 300x250 ad. This is to help reduce the impact of the loss of revenue caused by removing the two large ads up top.
Overall, I believe this will improve the sites appeal and therefore affect its longer-term growth.