Amounts of Resistance/Repetitions

You can change up amount of resistance and number of repititions also. For example, one week you might lift heavier weights in a lower rep range (say, from 8-12 reps per exercise). Then, next week you might lift lighter weights in a higher rep range (say, from 10-15). Or, do 8-12 reps with heavier weights for some exercises and 10-15 reps with lighter weights for others in same workout. Then, you can reverse it next workout.

Order of Exercises

You can change up order of exercise you perform from week to week as well. For example, on weeks 1-2, you start with lower body exercises. Then, on weeks 3-4, you start with upper body exercises. Or, one week you start with ab exercises and next week you end with them.

Equipment

You can also replace out equipment for exact same bodyparts. For example, you can switch out free weights for machines (i.e. do flat bench dumbbell presses for a few weeks and then use a seated chest press machine for next few weeks). Or, you can switch out compound joint movements (both joints working at once) for single joint movements (one joint working at a time). For example, do camber bar bicep curls for a few weeks and then seated alternating arm dumbbell curls next few weeks.

Tempo

Another fun way to change up your circuit training routine is change tempo by which you perform an exercise. For example, one week you can perform each repitition super slow and next week you can perform each repitition relatively quickly. I like using tempo counting method to determine exact tempo speeds. For example, a 3:1:3:1 tempo for a flat bench dumbbell press would look like this: take 3 seconds to lower weight, hold for 1 second at bottom, take 3 seconds to push it up, and then hold for 1 second at top before lowering it again. So, you could do a 3:1:3:1 tempo for a few weeks followed by a 2:2:2:2 tempo next few weeks. Or, use different tempos for different exercises within same workout and then reverse tempos next workout.

Circuiting Circuit

You can even create mini-circuits within a circuit training routine. One good way to do this is to group exercises for complimentary bodyparts together into their own circuit. For example, group all your chest, shoulders, and triceps exercises together in one circuit by resting 15 seconds between those exercises. Then, once you're done with them, take a 60-90 second break and go into another circuit for another group of complimentary bodyparts (back and biceps for example).

There are literally hundreds of ways you can tweak a circuit training program to keep it constantly fresh and challenging. And, as above suggests, you don't even have to make huge changes to get great benefits and results.

Word of Caution Circuit training can be very demanding on body. Before you start a circuit training routine, be honest with yourself about your current level of conditioning and keep that in mind when designing your program. Start slow and gradually increase intensity over time as your conditioning improves.

Conclusion So, if you need a program that is easy to update and keep fresh yourself and/or if you are finding it hard to fit in your workouts into your busy schedule, I highly recommend trying circuit training. It can certainly be a fun, challenging, beneficial, and time efficient way to meet your fitness goals.