There are lots of valid points from all the guys who responded. Sicilian, try new exercises for the same muscle groups. There are so many differen't exercises that if you keep switching constantly your muscles are forced to adapt to the strains and therefore are forced to grow...believe me...if you've always done bench press as a basic chest exercise and then one evening (I train in the evening) you do fat or incline dumbells instead, I can garauntee you will feel it the next day.

Pete