The first thing to do is get a big trash bag and start discarding any expired medication, both prescription medicines as well as over the counter medicines. You might be surprised at how old that OTC cough medicine is that is pushed to the back, or that cream which is now more like a rock because it has hardened as it is so old.
Next, make a list of medications that need to be replaced. If they are prescription medications make sure that you still need them and that they have refills or call your doctor's office with your list. Remember it may take a few days to get the medications replaced.
Routinely used OTC meds should also be replaced. I like to keep things like acetaminophen, ibuprofen, allergy medications like Benadryl or Claritin or Allegra, antacids, antibacterial cream and some steroid cream on hand. The medicine cabinet should also always have a working thermometer and it is not a bad place to keep a working flashlight as well. Always need that to look in a sore throat or a cut right? Maybe need extra batteries as well.
Lastly, with children of all ages in the home the medicine cabinet needs to be both out of reach and locked. The best way to prevent accidental poisonings or ingestion is by having a locked medicine cabinet.