If you do not have ammunition and you find two guards, sneak up to one and take him hostage. Make sure that your hostage is facing the other guard or he will shoot you.

If you do not have ammunition, hide in the shadows and wait for a guard to come up to you (or slowly creep up to him) then press R1 to punch him. Use two punches to knock out someone.

If you need to knock out many guards without killing them, look around for a water puddle. Try to lure them into it, then shoot a Sticky Shocker into the water. This will knock many of them out. This works with any body of water, including the sewers in the Chinese Embassy level.