A lot of people are in a similar boat Paul. It does seem to be a pattern where impressions increase and CPM falls. I can't tell you why exactly that is but it does seem to happen across HP.
In terms of how to increase your CPM that's a little easier to answer. The CPM is based on what you are writing. If you are writing about highly competitive terms that have high click values then you're CPM will increase. Examples of this would include financial terms (eg insurance), health related terms, weight loss etc. There are many more examples and you can check the rough values by checking the Google keyword tool. If you are lucky or skilled you can find keywords that have high click values and low competition.
If you are writing about low value keywords such as entertainment (movies/tv shows etc), personal experiences not related to any of the major niches such as the ones above, or other less commercialized topics then you're CPM will be lower. However, you can sometimes ofset this by getting lots of traffic for lower value keywords.