What you can do really depends on how much of your sites "visual appeal" you're prepared to sacrifice in order to attempt to raise your Adsense earnings.

For many, a 336x280 ad block at the top of the actual main content area provides a better CTR than many other ad formats, so that's something you could at least try by switching your existing ad blocks.

Many times, having ads coloured in a way which makes them stand out rather than blend in can attract more attention and hence more clicks. But again, this is often detrimental to your sites visual appeal.

It's a bit of trade off. As a webmaster, if you've spent a lot of time growing and working on your site, you can be reluctant to compromise it's appeal by more aggressive thrusting ads in your visitors faces. Not to mention your not wanting to give Google the impression your sites main function is to get users away from your site ASAP... but so long as your content is good and provides value for your visitors and/or answers their queries, you have nothing to worry about.

At the end of the day, if you want to maximise your earnings using only the existing traffic you have (without pumping up traffic numbers), this is likely the only way.

Having said that, if your ads aren't particularly closely relevant to your content and the visitors search query, you're unlikely to see much of an increase in CTR whatever you do.