I almost missed a connecting flight in Athens because I had gone to the wrong terminal. I turned up nice and early, found out that I was in the wrong place and stood there for a long time waiting for a bus. And it goes without saying that you should leave in plenty of time. You could also check the traffic report before you go, on teletext, the Internet or wherever. An international flight requires you to turn up two hours early.
Domestic ones are forty-five minutes.
Personally I like to be there a bit before that. It means that traffic problems or whatever won't make you late, and the time soon goes at an airport. In any case, it's good to have a drink, read the paper, do some shopping, and so on. Recently, lab tests have proven that blankets and pillows are far from clean even though they look fresh. You could end up catching something terrible that could ruin your trip.