I've always had really strong nails but in the last couple of years they have become weaker. Does anyone have any ideas as to what could be causing this? What can be done to improve weak nails?

Nails can sometimes be used as an indicator of health. My first suggestion would be to consult with your doctor to be sure of the exact cause. If it is indeed just a typical weakened brittle nail; take a look at conditioning nail products developed by pharmaceutical companies such as, Elon.