Following Google's lead, the other major search engines, Yahoo and MSN, have started weighting placement in the search results with your domain expiration.  Basically, if your domain expires every year, your placement will fall in the results since you are not planning on a long term existence.  Even though this might not be your intention, it appears so to the placement algorithms. 

The placement weighting for domain expiration can be as high as 75% of the overall value.  This level definitely warrants extending your domain expiration to 5 or more years into the future.