Changing Gears

[zenmushin]

I have been playing for about a year and a half and in the last couple of months have been cashing in tournaments on a regular basis.
However, it seems that as the tournament reaches the later stages and even at the final table I am rarely in contention to win. I seem to accumulate a strong chip stack early, but have trouble adding to it. My stack gets smaller to blinds and eventually antes and I go from a strong stack to survival mode. I don't feel that I change the way that I play which I think is costing me as we go deeper into the tournament.
I am looking from some adivce on how to keep building a stack and how to "change gears". I hear people talk about it, but don't understand when to do it and how that affects how you would play.