Ha ha ha… You nailed a very Serbian topic right on the head: expressing opinions. That's the only thing that Serbs like more than drinking coffee and smoking! You better get your magic vocabulary powers on for sparring in that game !!! :)
Here are some off top of my head. You can (or should) usually add "da..." to the end as in "that..." in order to continue the sentence:
I would say that... - Rekao bih da...
I think - Ja mislim
I am convinced - Ubedjen sam
I agree - Slažem se (rarely used as they find it hard to agree :)
I disagree - Ne slažem se (you're a Brit, it's expected to be polite :)
I can't say I agree (with you) - Ne mogu reći da se slažem (s tobom / sa vama)
The ones that you will most often hear from them are:
Da ti objasnim ja nešto - Let me explain something to you
Sad ću ja da ti objasnim - I'll explain it to you right now
Da ti kažem ja nešto - Let me tell you something
Slušaj, brate, nemaju oni pojma - Listen, buddy, they have no clue
Ma šta oni znaju - What do they know
Ma šta lupaš? - What are you blabbing about? (lupati - talking nonsense)
Slušaj ti mene - Listen to me
Ma kakvi - No way (not correct, not true)
Nije istina - Not true
To nije istina - That's not true
Šta, bre? Ma jok! - What? No way! (with Turkish seasoning)
Šta znaju Ameri? - What do the Americans know? (meaning nothing)
The best ones use some juicy curse words but I'm afraid this site is not an appropriate place to share them! Hope these help and they don't get you in any fights you wouldn't otherwise be in :)