Remaining hidden involves five things to remember. Those are:
Shape - Avoid hard lines, few things in nature have right angles or straight lines. Disguise obvious items like guns, radio antennas, helmets, etc. The Ghillie suit excels at this function.
Shine - Any reflections will give you away farther than anything else. Tape over power LED's, and watch out for cellphone screens, and such. Gloss items are your enemy, if push comes to shove, burn a cork to get carbon to blacken items. Check weapons for places that wear and either paint them or tape them over.
Silhouette - Be aware of your surroundings, do not walk along a ridge line, as the sky behind you makes you stand out as clear as if you had a neon sign over your head. Likewise, be careful when moving across light colored backdrops. A grove of white birch, a pale outcrop of rock, or a snowfield is just as good at giving away your position.
Shadow - Be wary of light overhead, the shadow from a full moon or the sun adds to your size, which increases the chance you are spotted. Plus, the black blob stands out starkly on the ground.
Spacing - When moving as a group, spread out. the bigger you are the more likely you will be picked up by an observer's peripheral vision.
I am a big fan of the Ghillie Suit. I played many a game of paintball where the opponent got within 5' of me without their knowledge. A particular nasty trick of mine is to wait at their flag station, and mark their flag carrier when he was almost to the flag station. When they had to walk the flag all the way back to my station, we called it "the walk of shame."
A full ghillie suit is not needed most of the time, add strips of cut up BDU's and sew or tie them onto your boonie hat. That will break up your outline when peaking out of cover. Most snipers use a cape and leggings that only covers the back of their body. This way their head, back legs and feet are covered, but if they need to haul ass they don't have a giant bulky suit getting in the way. Commercial suits are available too, this is Cabelas' version.
A good thing to have is chicken wire. Stuff local vegetation into it and you have a mat of the perfect camouflage for the local environment.