For night play:
Ghillie's are incredible camo at night. They are effective during the day, but at night they are nearly impossible to detect when used properly. This means you will need to take some of the local environmental cover, and integrate it into your suit.
Several months ago at a scenario game, a friend and I tried our ghillies for the first time. We had silently crept to within 20 feet of some opposing players and taken them by complete suprise. We continued along the wooded edge fo this huge open field with speedball bunkers in the very middle of it being defended by more of the opposing team.
The people putting on the game repeatedly fired flares over the field, and swept it with floodlights every few seconds. We would crawl on our bellies untill we heard a flare launch, then quickly cover our markers with our bodies and keep the lenses of our masks pointed down to avoid throwing a reflection. After a good half hour of crawling inbetween flares and spotlight sweeps, we were again within deadly range of our opposition. Again we took them by complete suprise and eliminated at least six before they broke and ran. Finally, as we were lying still in the grass deciding where to go next, we heard voices approaching us saying "hit... hit... hit...".
It was two eliminated player from the opposing team. They continued towards us and I was very afraid they were going to give our position away despite the fact that they were "dead" from a game standpoint. Just as these thoughts were going through my mind, one of the players foot brushed mine as he stepped over the pile of leaves that was me. They continued on for a few more yards when I heard one of the players exclaim, "Those were people back there!"
I learned a few things after that game. Ghillies can be very hot, you need a fan for your mask to reduce fogging. You want a lightweight marker with a small profile that you can crawl around comfortably with. Finally, trust your ability to remain nearly invisible when wearing a ghillie in suitable surroundings. The opposition will walk right up to you. If you carefully pick your shots, and use distractions (like throwing a stick to draw attention away from your position you can remain in one place. If you can't fool them into believing the shot came from elsewhere, move quickly, quietly and immediately after you shoot. Then be still again and wait. They likely will lose any concept of your position again, and in time another opportunity for a shot will present itself.