Spotting problems before they get worse

• Watch for leaks or stains under the car
  If there is a persistent red oil leak that you are sure is coming from your car,  you should have your shop check to see if it is coming from your transmission or possibly from your power steering system (most power steering systems also use transmission fluid and leaks can appear on the ground in roughly the same areas as transmission leaks.)  If all you see is a few drops on the ground, you may be able to postpone repairs as long as you check your fluid level often (but check with your technician to be sure.)  If transmission fluid levels go down below minimum levels serious transmission damage can occur (the same advice goes for power steering leaks as well.) 
• Check fluid for color and odor
  Most manufacturers require that you check transmission fluid levels when the vehicle is running and on level ground.  Pull the transmission dipstick out and check the fluid for color and odor.  Transmission fluid is a transparent red oil that looks something like cherry cough syrup.  If the fluid is cloudy or muddy, or it has a burned odor,  you should have it checked by your technician who will most likely advise you to have a transmission drain and refill or transmission tune-up. See the Maintenance section below for details on this service.

Maintenance

Transmission fluid should be changed periodically.  Your owner's manual should give you the recommended intervals which could be anywhere from 15,000 miles to 50,000 miles.  Most transmission experts recommend changing the fluid and filter  every 25,000 miles.

Few transmissions have drain plugs to drain the old fluid.  In order to get the fluid out, the technician removes the transmission oil pan.  This is quite a messy job and generally not recommended for the casual do-it-yourselfer.  Even if the transmission has a drain plug, the only way to also change the transmission filter is to remove the pan. When the pan is down, the technician can check for metal shavings and other debris which are indicators of impending transmission problems.

In most cases during these transmission services, only about half the oil is able to be removed from the unit. This is because much of the oil is in the torque converter and cooler lines and cannot be drained without major disassembly.  The fluid change intervals are based on the fact that some old fluid remains in the system.

When the transmission is serviced, make sure that the correct fluid is used to re-fill it.  Each transmission manufacturer has their own recommendation for the proper fluid to use and the internal components are designed for that specific formula. GM usually uses Dexron, Fords prior to 1983 use Type F while later models use Mercon.  Late model Chrysler products use ATF +3 +4 (Not using the correct fluid for Chrysler transmissions is the most common reason for their transmission problems.)  Toyota sometimes uses Type T which is only available through Toyota and Lexus Parts departments.  Honda also specs out their own formula which is available from Honda or Acura parts departments.   A transmission will not work properly or may even slip or shudder with the incorrect fluid, so make sure that you double check.  Your owner's manual will tell you which fluid is required.  Naturally, the owner's manual will try to convince you to only use the manufacturer's branded fluid, but they will also provide you with the specs for the oil.  If the aftermarket product indicates on its container that they meet or exceed the specs for a particular type of transmission fluid, it is generally ok to use that product.