My power steering is fine when I start the car in the morning and drive for some distance, then after awhile it'll get tighter and finally becomes like a non-powersteering car. When I take a full turn when the car is in idle, it turns extremely hard, but when I give the car gas it starts to turn more freely, respectively. My car is a Honda City 1.5exi 2001 model.
If the belt isn't squealing (indicating slippage), and the fluid isn't low, and there isn't any grinding (from rack&pinion system), then it sounds like the power steering pump is failing as CharlieRB suggested.
Check that the fluid level isn't low and that the drive belt is tight.