i always do my core exercises to start off with but whenever i go to finish up a muscle group i like to switch it up and do different exercises that keep my workout interesting.
try doing pushups with a twist(barbells in each hand, go down, back up, then twist your body as you extend one arm straight up, then bring it down and raise the other arm. then repeat. it's a good one for the rear shoulder.)
the reverse pushup is good(hang from a bar that's a few feet off the ground, with your heels on the floor, pull your chest up to the bar. works best if you contract at the top)
reverse grip curls work your bi's at a different angle so that you don't have too much of a peak.
the best thing you can do is go out and buy magazines about fitness that have diagrams of different exercises. find the ones that look interesting and give them a try in the gym. just keeps you from getting bored with your routine.