I have had a two-level spinal fusion (l5-s1). what is the best way to avoid (or prolong) adjacent disc disease?

Not much : Accelerated adjacent segment degeneration is relatively common and concerns adverse effects following spinal fusion. It is believed that this accelerated degeneration is due in large part to increased mechanical stress on the adjacent motion segments. Stay in shape, do not gain weight.

Fusion: I think the best ways to help minimize that risk are to stay active with low-impact activities such as swimming, cycling and to maintain good core strength. It is also important to avoid jobs that require a lot of repetitive heavy lifting as that will ultimately cause more "wear-and-tear" of your back.

Not Well understood.: I agree with the other providers but the reality is that "adjacent level disc degeneration" is not well understood. The stress from fusion may accelerate this by about 3%/year. However even with patients who have disc replacement over fusion get this condition as do patient who have simple laminectomy. Thus, there is likely a genetic component to who develops this and who does not.