1. USE YOUR SUIT!!!! If you're not doing anything cloak, are at least activate your armor. It's a waste of energy if you don't use it.
2. When sniping, hold still and cloak until you're ready to take a shot . This way, no one will see you until you light them up.
3. Maximum armor is very useful when taking long falls. Use it before you hit the ground to avoid taking massive damage.
4. Cloak when you are behind someone holding still. You can then one-hit silent kill them. If you melee them twice, it's possible to get killed in some unfortunate manner before you secure your kill.
5. Use your weapons effectively. Don't run around with a sniper rifle, and don't try to snipe with a shotgun. It may seem obvious, but it's surprising how many people try to use weapons differently than intended.
That's all I have for now. Let me know some other tips if you have any!