With a little practice, you can master the art of having both of your children holding one of your hands, with one child walking in front of the other. This is invaluable in freeing up your other hand to carry other things.
Give each child a space of their own. This doesn't have to be anything big, for my girls it is just a drawer where they can store their treasures and things that they designate as just theirs in a world where they share virtually everything - toys, clothes, room, birthday, etc. For us, this ended a lot of arguments.
Make copies of partially filled out school/camp forms to prevent having to fill everything out twice. Filling out piles of forms in duplicate gets extremely annoying. For many of these forms, the only thing different for the twins will be their name at the top. If so, you can save tons of time by filling out one form with everything except the name, then photocopying it and adding the names in.
Odds/Evens. One of my friends had done this with her two boys, and I thought it was genius. Assign one twin to odds and one to evens (we did this based on birth order so it would be easier to remember). When things come up (who gets to go first in line, pick the movie during a car trip, etc.), it is decided by whether it is an odd or even day of the month. This has eliminated numerous arguments about whose turn it is to do things, and as an added bonus they have learned the concept of even and odd (fortunately, they have not yet realized that there are a few extra odd days each year...).
"If you cut it, she gets to choose which piece she wants." This was the rule in my house growing up and works especially well with twins for whom sharing is a way of life. If something needs to be split in half, one twin divides it and the other gets to pick the first piece, ensuring that the splitting is done as evenly as humanly possible.
The 3 person playdate is difficult to navigate. It seems like someone is always the odd man out and getting their feelings hurt. Even though it is more people, I found that it was often easier to have 2 friends over. If your kids befriend another set of twins or siblings, that makes it easier.