I found this method from an aquarium hobbyist website. The main purpose of that article was to culture protozoa to feed the young fry. I found this to be a very effective way to increase the population density of the microorganism. This allow us to find our tiny creatures easier.
What you will need: a half to one gallon container, bubbling stone and aquarium tubing, aquarium air pump and some scrap food (any vegetable will do but I like to use carrot peel the most.).
How to do it: Put some carrot peel into the container then fill up with water to two third full. Collect some pond water and innoculate into the container. Aerate the water with bubbling stone by connecting it to the air pump with aquarium tubing. Put it in a warm and sunny place and watch it turn turbid. You may want to take a few drops of water to observe just to see the change.
The first few days, the water is crystal clear and very few organisms are found. In about two week, my culture turned turbid. The number and variety of the pond lives in the “carrot soup” is amazing.