This seems so obvious I almost hate to list it. But like many "obvious" things, I never once thought of it until a friend suggested it.
Two of my son's friends from nursery school are fraternal twins. Their mother is very careful to only rarely put them in totally matching outfits, but when we go on playdates and it's not just the twins but the little one too, she is sure to put all of them in the same color shirt. This makes it easier to find them on the playground; once she remembers the "color of the day," it's easier to scan for that color and then the face rather than the other way around.
So why is this the '80s flashback? I was fortunate enough to be the recipient of almost an entire boys' wardrobe, from infant to size 5, when Son 1 was born. (Alas, when the donor cousin started wearing size 6, much to many peoples' surprise, a brother came along and the hand-me-overs stopped cold. Bummer, but understandable.) Not only was I lucky that the clothes arrived to begin with, but the cousins who provided them have excellent taste (by which I mean, our taste in clothes for the most part overlaps). Of course, their son apparently would wear overalls, which mine will not, but otherwise, it has been an amazing gift to us.
However, whoever bought these clothes was clearly very happy in the '80s (as was I). A shocking number of the pants (for a boys' wardrobe) are bright yellow (think construction hats) or fluorescent orange. I wouldn't usually pick these. But, when we are going somewhere crowded, they are my first choice. It makes them easy to find, and (G-d forbid) easy to describe if one gets away from me. It has saved me time and time again from needless worry, and has me putting the boys in different clothes than I or they might normally pick, allowing the wardrobe to expand a bit.
That's what works for me! For more (and probably more original!) ideas, go see Shannon's blog at Rocks in my Dryer.