impressions / number of clicks = CTR (click through rate)
(total earnings / impressions) x 1000 = eCPM (estimated cost per thousand impressions)
To make them increase, you basically need more clicks on your ads OR, less total traffic and keep the number of clicks you have. Note that the second option usually sucks because your earnings won't ever grow. More traffic & good content will insure that the ads stay relevant to your page.
Be sure theree aren't too many options for your users. You want: "read, read, read. Hmm, now what *click*"