Birkenstocks are meant to be worn loosely. Support comes from the footbed -not tightly secured straps. Wearing them loosly allows your feet to move naturally. It also improves circulation. Backstraps worn to tightly push the feet forward, preventing them from resting naturally in the footbeds.
Some people find Birkenstocks immediately comfortable.
For others, it takes a few hours a day for a week or two to break them in. Although the shoes may feel ridgid at times, remember that the firm support is essential for the orthotic benifits of Birkenstocks. They will become more flexible with wear. The uppers will become more supple and the footbeds will adjust slightly to your feet. If your feet are unaccustomed to the contour footbed, start gradually.