I can't remember whether I read or heard this idea somewhere or I actually thought it up on my own. Adam and I started doing this with our daughter when she was old enough to be out of her stroller during our visits to the local aquarium (where we used to have a membership, so we went quite a bit).
Before heading out of the house, take a couple of digital photos of each child. Make sure one shows the whole outfit (or two if coat/hat/etc. will be worn--one with and one without) and another is a close up showing facial features. If you get separated from your child(ren), you not only have a recent photo available, but one which actually shows the clothing they are wearing.
For more tips, tricks, and ideas, visit Rocks in My Dryer.
**Edited to note: The instant I published this, I heard a crash from my daughter's bedroom. She'd fallen out of bed. Apparently she was calling my bluff on this safety business!