#1 Lower Water Level. Try not to fill your aquarium all the way to the brim with water, as water drops down into the aquarium (waterfall effect) from the filter it forces water down into the aquarium and helps push oxygen into the water thus helping to increase oxygen levels (be sure the current is not too strong for the aquariums inhabitants).
#2 Water Changes. Too many water chemicals and/or waste, etc can cause a loss of oxygen in the water, it's very important to do regular water changes (clean tap water contains adequate amounts of oxygen).
#3 Lower Temperature. Cooler temperatures can hold more oxygen than warmer temperatures but, be sure the aquarium is the proper temperature for the aquariums inhabitants.
#4 Air Diffusers, Bubblers, etc. With these devices attached to an air pump it helps to increase oxygen and water movement in an aquarium.
* Bubbles should be tiny (almost pin hole size) looking for the most part, and probably vary in size. The tank should not just have large (about a cm) and/or slow moving bubbles as this may mean your water has become thickened by dirty water. Also be sure your air pump is strong enough to operate the equipment properly, and clean or replace old diffusers, tubing, and check valves as needed.
#5 Filtration. The right amount of filtration is key to keeping a healthy and clean aquarium, you want to make sure the tank is not overstocked with fish either as the fish will be competing for oxygen. Keep filter cartridges clean, and active carbon can help remove unwanted chemicals and aid in oxygen levels.