I will also be doing my first milsim this summer, and from what experienced milsim players have explained to me, there are 5 important things to keep in mind (most of which people have already mentioned here).
1-water and lots of it, carry some and leave a lot back at the CP.
2-extra clothes, including something for if it rains.
3-don't wear cotton underwear, wear synthetics that wick away moisture (under armour, exofficio, etc.).
4-food, you will need plenty of calories.
5-do not show up out of shape.
Obviously there are many more things to consider but those 5 elements should make a difference as to how comfortable you will be during a milsim.
I am going to rely on my portaging/canoeing experience for comfort, and I am going to rely on listening and following orders for the military aspects.
Can't wait until the milsims start...
A Canadian is someone who knows how to make love in a canoe - Pierre Berton