One of the most important things in fitness is to keep the body adapting. Too often, because of the popularity of certain programs, people get locked into the routine of 3 sets of 12 or 5 sets of 5. This causes many people to plateau and not understand what they're doing wrong.
In order to keep adapting, you need to constantly change variables to keep the body guessing. I see many people go into the gym and hit a bech press for 3 sets of 12, then flye machine for 3 sets of 16 time after time again. This is okay as an amateur lifter, but after a while the body knows what is coming, and is prepared for it. It's generally reccomended that you do a workout no more than 3 weeks in a row, as that's about how long it takes for your body to adapt to a particular stimuli.
In order to break free from this paradigm, we merely have to think about exercise a little differently.
Why not try doing a medium-heavy weight on a particular exercise with the goal of 50 (or 100) reps in as short of a time as possible? Do 8 reps, set the weight down, take a breath and continue as quickly as possible. This pushes you past the point where you would normally stop when doing 12x3. Then you can try to beat your previous time in the next workout. The key here is intensity. It is necessary to use different intensity techniques in order to assure your workouts stay consistently difficult. Another common technique is doing static holds at the end of a set, which extends the time under tension. This is a key variable in building strength and mass. There are many different ways to do this, basically limited only by your creativity. Some people like doing 1 1/2 reps, where for every rep completed you would do another half rep before counting it. A popular bicep workout 21s, where 7 reps in the lower half of the range of motion are completed, then 7 in the higher half, and finally 7 full range of motion repititions. One of my favorite techniques is using forced reps. Take an exercise to failure. Then go past it, having a partner assist the weight up, afterwards lowering it down as slowly as possible without help.
Further still, I've talked to trainers who like to count by the total amount of weight lifted in a workout. For example, if you do deadlift 3 reps at 315lbs, you lifted 945lbs on your way to a goal of several thousand.
The point I'm making here is to get out of your comfort zone, so that you avoid spinning your wheels in the gym. This in turn will increase results and satisfaction by leaps and bounds. Good luck!