There are many ways to help this problem so I'm going to state some of the more useful ones. (Those of you who have gone through anatomy/biology classes in college or possibly even high school can skip the first paragraph if they want

.) If you do not care about the science behind this stuff you can just skip to the bottom where I will give a short summery of what to do to keep cool
.
First off I will tell you how your body regulates its temperature. Humans generate their own heat through the chemical and physical reactions going on throughout our bodies. However we have also adapted to be able to release excess heat for when our bodies' core temperature is risen to above normal levels (98.6*F). When we overheat, our brain redirects the flow of our circulatory system so that our blood flows as close to the surface of our skin as it can. This is why people look red when they get hot. When our blood runs near the surface of our skin it allows our internal heat to easily pass through the skin and be released to the surrounding environment. Our skin also has pores that can open and close to either increase or decrease the speed that our heat is released. However, what could be considered the most effective adaptation that humans have to regulate heat is our ability to sweat. Sweat is a combination of water and salt that our bodies release to quickly disperse heat. The sweat absorbs our bodies excess heat until it can't absorb any more. When it reaches its limit, it evaporates off the skin taking the excess heat with it and therefore cooling off your body by releasing excess heat.
JUST SO THAT IT IS KNOWN: The spots that release the most heat in your body are:
YOUR HEAD
NECK
ARM PITS
GROIN AREA
Keeping these areas covered in a lot of layers makes it very hard to release heat. In short the thinner the amount of material on these areas
the cooler you will stay.
Now that I've said how your body naturally regulates heat I can tell you what you can do to make the best of what it can do alone, along with telling you a few ways to help cool yourself off by other means.
Cold water is your best friend in lots of heat

. I've played my fair share of 90 degree games, though I didn't have a ghillie, and I've found that soaking your hair and whatever is around your head in cold water always helps. Drinking cold water helps as well. If you have time in between games take off your gear and dowse your body in water then dry it off with a towel.
The science behind the cold water is that water needs to absorb more energy than air and other materials to change temperature. Colder water allows even more energy to be absorbed. Water also has the lovely effect of evaporating off your body once it absorbs to much heat. When the water evaporates it takes the heat with it. As stated, sweat has the same effect but it does a better job of absorbing the heat because of the salt contained in it. When salt is added to water it increases the boiling temp for the water. In short, sweat can absorb more heat before evaporating off your body.
Now a large problem with the ghillie suits we use is that there aren't many breathable spots in the fabric of the suit for the evaporated water to escape through. If this is the case you can soak all of your clothes with cold water if you want, but you will still feel just as hot because there is nowhere for the heat to go

. If you cut several slits in the fabric and loosely sew them together so that they stay open but don't rip more, you can help the evaporated water escape with the heat it took off your body. Only once the evaporated water escape will you begin to feel cooler.
Another way to help keep you cool is to always wear some kind of wicking under layer. The military always provides their soldiers with some basic shirts and socks that are made of lightweight, breathable materials. Wicking technology basically takes the sweat away from you skin and spreads it out allowing it to evaporate faster. This process releases heat quicker and leaves you feeling drier.
As Fuzzy mentioned, whenever you feel overheated,
slow down, and there is actual science to back this up. When you move or exert yourself your body makes more heat. If you start producing heat faster than you can release it then you will eventually overheat and pass out. By slowing down and stopping occasionally you won't produce as much heat and will give your body a chance to cool off. Avoiding the sun should speak for itself.
Any time you can, get a breeze to blow through your gear. It will also do wonders. As wind blows over your skin it quickly picks up heat being released by it and carries it away.
So just to summarize what you can do too help keep yourself cool:
1) Drink cold water. Colder is better because it absorbs more heat and replaces what you lost from sweat.
2) Soak what you are wearing in colder water. Especially things around the head and neck. You can also soak the other clothes you are wearing.
WARNING: DO NOT SOAK CLOTHES IF THERE IS LOTS OF WIND OR COOLER TEMPERATURES! Feel free to soak a hat or a balaclava, but if you over do it in a cooler or very windy situation you can get hypothermia. Just use common sense when doing this.
3) Slow down and find some shade if you feel like you are over heating. The slower you go the easier it is to cool off. If you do begin to camp out somewhere, try to choose a shady place or at least make sure you don't camp in a pocket of hot air. As Fuzzy said, taking a break now is better than being done for the day because of heat stroke.
4) Make sure there are holes or breathable fabric for evaporated water and heat to escape through in your ghillie or gear that covers your whole body. If there is not, cut and loosely sew holes into the ghillie/gear so that the heat can escape. Most BDU, ACU, CAMO, etc. pants and jackets are breathable so there should be no need to cut holes in these.
5) Try to let a breeze pass through your gear to take away some of the extra heat quick. Plus it just feels really good

.
6) Wear wickable fabrics for base layers. These will make you feel much more comfortable by keeping you dry and will help keep you cool in the long run.
7) Try to not have any more layers than just your base layer and your outer layer. As far as ghillies go, just wear your base layer then your ghillie. Try not to wear vests or anything under. Battle belts and suspenders should hold all of your stuff and then you just have your Camelback.
If you do all of these you will feel a lot more comfortable. Every person is different so adjust what you have to do as necessary.
Hope this helps and remember, stay cool.