i find it hard at times too, especially when it's an intense discussion and everyone is trying to get a word in, but with time and practise you can get better at getting your points in. i can't really give you any specific pointers, but when you're in a group try to observe the others in the group and how they cut in. try to see what tells them the other is done talking, or if they're interrupting, try to figure out how they do it without angering the others. and i agree, it's not always considered a bad thing to interrupt if you have something pertinent to say. if you cut someone off and don't really say anything interesting or don't add to the discussion they're more likely to dislike it.