Some airlines will extend the life of your miles if you spend more money with them. Some options are: transferring miles to another account. The airline usually charges a small fee for this. I
've also heard that purchasing products with your frequent flyer miles will extend the life of the remaining miles. Magazine subscriptions can be purchased for relatively few miles.
These solutions to the mile use by date issue are not well advertised. After all, its in the airlines interest that you lose your miles...The options are however listed on most airlines' websites.
Frequent Flyer fourms is another great venue for getting advice on how to extend the life of your miles witha a particular airline.