O2 to the roots is a big concern, since the plant requires this for nutrients to be available, and to rid itself of toxins, etc. One of the easiest things to do is use food grade hydrogen peroxide in the water to increase the availability of oxygen in the water. h1O2 has an extra oxygen atom that will easily break away and can be used by the plant. Oxygen Plus is a plant food that contains 25% hydrogen peroxide and is perfect for this use.
Using a planting medium that allows for plenty of aeration is also really important. Be sure you have good drainage by using Perlite, sand, or gravel in your mix and at the bottom of pots. Don not use a medium that holds too much water, or you may significantly reduce the oxygen available to the plant. More on that in the section on hydroponics.
Aerating the water before watering is also a good idea. In the case of soil potted plants, use an airpump to aerate the water overnight before watering your plants, or put the water in a container with a cap and shake it up real good before giving to the plants.