It should be easy to keep track of something as large as a septic tank, but in fact, the opposite is usually true. After several years of sitting undisturbed in your yard, the soil above it has settled and the ground cover effectively camouflages it, so finding one usually requires some detective work. Even after you do manage to find the tank, you still have to locate the cover before you can service it. Using the process of logical deduction and two helpful tools, though, you should be able to do it without wearing yourself out with a shovel.
Consult a site plan for your property that specifies the location of the tank. If you don't have one on hand, you can look it up in the records of the county building department where the contractor who installed it was required to file it. Note the relative orientations of the tank and your house and the distance of the tank from the side of your house from which the sewer exits.
Locate the sewer clean-out on the side of your house and measure the specified distance in the direction of the tank. Begin probing for the tank at that point by driving a 6-foot length of re-bar into the ground with a sledge hammer. As soon as the re-bar encounters an obstacle, stop hammering and excavate a foot or two further on. Continue doing this until you can drive the re-bar deeper, signifying you've reached the end of the tank. Find both ends of the tank in this way and mark them.
Run a metal detector over the area you marked out to find the cover. It is usually metal or at least has some metal parts. If the tank has an effluent pump, which is always located under the cover, the metal detector will also sense that. Begin digging at the place where you get a positive reading.