Install low flow showerheads
Low flow showerheads are now designed to provide an effective shower with much less water than an old showerhead (52% saving) or even than a new standard showerhead (20% saving).
Look for WaterSense® certified showerheads. Numerous versions are available at most hardware stores. If you plan to DIY, check out page 37 of the CMHC Household Guide to Water Efficiency.