To score a hundred is a fine achievement at any level. I have seen guys score lucky fifties, but to score 100 is an effort, no matter the level.
In all the tons I have scored, there have been a few common themes. All bar one have been scored with a gradual progression and acceleration throughout the innings - the other I just had one of those days where everything clicked (the first 50 came off 29 balls). Concentration is really the key, and the realisation that once you get to 40/50 then generally you are in, and unless you get a great ball, the only way you will get out is if you make a mistake.
Once you get to 90 try not to fall into the trap of changing the way you are playing just because you are close to a hundred. Try to relax and just continue to play. Forget the scoreboard. You have got to 90 - the runs will look after themselves.