I have mixed results on this.. One forum is at 27% while the other is around 55%.

What type of rates are you seeing?

Are you getting them figures from Google Analytics. If so then I don't know mine as I don't just track the forum alone, but the whole of the site as a whole. My bounce rate for the whole site as an whole is in between both of your figures but slightly closer to the lower one.

What are good ways for bringing down those bounce rates?

depends - but ensure that landing pages offer relevant content to the visitors..

high bounce rate can be related to many factors:
1) CPC ads not matching the actual content of your site
2) Meta description not matching the actual content of your site
3) too much advertisement on the pages, making them look like spam pages
4) etc ..