adding more ice is really the only way to make the water actually colder. but for sure moving your legs around or circulating the water sure will make it feel colder.
smaller chunks will cool the water off faster, but larger ones will keep it cool longer. the same volume of ice will cool it to the same temp, but just alter how fast it gets there.
ive heard 50-55 degrees is the ideal. anything colder isn't any more productive, and you can risk negative side effects too.
i always liked to wear sock folded down around my toes to keep them warm, otherwise it can be pretty brutal for the first few minutes until you completely lose feeling in them.