Optimising Adsense Clicks within vBulletin

[MrEyes]

I realise that optimising Adsense click rates verges upon being a dark art that requires summoning of various internet deities. However I would like to get some advice from fellow VB site admins.

Currently I used Adsense on my site as follows:

Logged in users
1 x text only, 728x90 leaderboard in the header
1 x text only, 234x60 half banner after the first post

Guest / Logged out users
1 x text and images, 728x90 leaderboard in the header
2 x text and images, 335 x 280 large rectangle after the first post

These are displayed on all relevant pages with the exception of the vBAdvanced home page.

Regardless of logged in status these banners are contained within centred cell of a VB styled table, with a header of "Google Sponsors". In addition to this the banners are styled to fit in with the forum style.

Now all these design etc decisions were taken after spending days reading through the myriad of, often contradictory, advice out there.

So according to Google, since the beginning of April there have been just under 1 million impressions, however these have only generated 235 clicks. Which after reading some other click rates seems to be pitiful.

So where am I going wrong?