|Rather like the stock market, I find that the long-term trend with AdSense - as long as I keep writing and adding new content to my site - is upwards. |
Occasionally, for a month or two the earnings graph heads south a little way. Usually when it resumes it quickly passes the previous peak and continues to climb.
Experience has shown me that every time earnings start to drop it's more likely to be a short term (ie. one to two month) blip than anything more significant.