As others have mentioned, your routine should be built primarily on big, compound movements. I've found the best way to use isolation movements is after I've already done the compound movements. A set of bench press, for example, could be followed up later with skull crushers/nosebreakers later in the workout.
Some training philosophies call for doing isolation movements before a compound lift. The idea is to "pre exhaust" a muscle--like doing a set of flyes right before you do bench press. I've tried this but stopped--seems like the risk of injury is just too great.