So now that you know about prediction you have to learn methods to make you unpredictable. An armor advantage is fine, but in UT getting the 1st shot is more important in most situations.
It`s not that easy to be unpredictable, especially if you got control. You have to be at the shield/armor exactly at it`s respawn time, so that doesn`t leave you much room to do something unexpected. What you can do though is scout the area first. Especially if you still have enough armor, you don`t need to take shield/armor immediately, just guard it and search all the ambush places nearby. Picking up the shield doesn`t help if you get a headshot immediately after taking it. Sometimes the better option is to let your opponent take it, so he will be predictable. You should never go straight for the shield/armor if you don`t know where your opponent is or you will be wish you wouldn`t have taken it. This is why you should go for the shield/armor in time and without getting distracted too much.
You should also try to use different routes each time you go from armor to shield or back.
Faking is a very important tactic as well, just fake going somewhere, but take a different direction. You can for example make an elevator go up, but without you on it. Or you can go into a hallway, but turn around and take a different way, if your opponent saw you. Or you can start picking up health vials, but then retreat and wait for your opponent to come, so you are the one to surprise him.
For the underdog it`s very easy to use the surprise element, cause you know where the guy in control will show up and you should also have a rough idea, when that happens. Or you could just hide along one of the routes or feign death somewhere and then attack from the back.
There are hundreds of possibilities, show your creativity.