On a pond that size I think your already doing the best thing by using a slip bobber. It allows you to fish different depths and cover allot of water. With a good rod & reel you should be able to cast to most of the pond.
I've started using different color plastic tubes in the last year that have really helped me to catch more fish. Try black/chartreause, red/ chartreause these are everyones favorites. But also try some subtle colors such as pumpkinseed, rootbeer, watermelon red.
Tubes with Crappie Nibblets are sometimes more effective than minnows.
Sometimes you can catch crappie on colors that are almost invisible in the water but the crappie will see them and will hit them.
Good luck