You may feel like any kind of liquid can be placed down the system but things like chemical, oil or grease should not go in there, particularly if you have a septic tank. The garbage disposal is not intended for nonfood items either. You may feel this is obvious but its surprising how many things end up in the sink along with food scraps, think of items like aluminum foil or even bottle caps.
Some foods could also create problems with a garbage disposal unit. Harder items such as bones should not be placed down there, also foods that are particularly fibrous like corn husks and potato peelings can clog up the unit. If you really want to put them down there, do so in small amounts and keep the water running for a little while even after switching off the garbage disposal.
While these may seem like very simple suggestions, following them will do a lot to prolong the life of your kitchen garbage disposal unit.