Assign a color and seating order!
From the time the girls were born we always put Finley on the left and Gillian on the right. Why would this ever matter you ask?
Well, its simple really. Finley is a peanut, Gillian is not. They have always had a big weight difference in them so by assigning a "spot" for them early on you make sure you never have to adjust and readjust straps on things.
If I put Gillian in a car seat Finley used last we would have to loosen the straps to get her in. What a pain to do every time you're going someplace!
Same goes for strollers (if your using a side by side) and high chairs. Always putting them in the same spot ensures you aren't going crazy loosening and tightening straps back up to accommodate the other baby!
Second thing is colors! I'm pretty sure this extends beyond the "twindom" since I remember always having a color when I was growing up! That's what happens when you have 2 brothers.
Anyway, assigning a color early on helps out when you're trying to figure out who's toothbrush is who's or who's sippy cup is who's etc etc. Pick a color for each child and stick to it if you can.