A grounding sheet works because thin silver threads are woven through the cotton. As long as those threads stay clean and intact, the sheet keeps conducting. Most of what shortens its life comes down to three things: buildup on the fibers, corrosion and too much heat.
Keep the fibers clean
Body oils, lotions and fabric softener leave a film on the silver that slowly blocks contact with your skin. Wash with a mild, residue-free detergent and skip softeners and dryer sheets entirely.
Avoid harsh chemicals
Bleach and other strong agents attack the metal and make it brittle. A gentle detergent made for conductive fabrics is all the sheet needs.
Go easy on the heat
Wash on warm or cool and dry on a low setting, or hang it to dry. High heat stresses both the cotton and the silver.
Check it now and then
Use the continuity tester that comes with the sheet every few months. If the reading drops, a good wash usually brings it back.