My strategie is very simple. As I said I bought under 40 books since I got my first reader at the beginning of last August. That is ten month from now = 4 books per month.
I have a prepaid Visa card that I use for internet purchases. I just fill it up with my monthly playmoney and spend it on books, DVDs, girlie knick-knack, icecream or stuff like that. As I have been ill/out of job since Christmas, money is not flowing in like it did and this strategy works very well.
In those 10 month I build an e-library with nearly 1,200 books. All freebies, most classics (I was extremly greedy in the beginning and still am most of the time).
So get yourself a playcard and put your pocket money on it.