It's easily done, but one mistake you definitely want to avoid is not doing a pre-workout warmup. It might save you time by not warming up, but it's definitely false economy. The chances of picking up an injury are greatly increased if you decide to launch head-on into your workout.
If you are working on weights, the likes of lunges, pushups or jumping jacks are a great warmup activity, as they give your heart rate a boost. If you are planning to start your workout with a big lift, then a set with high reps but at a lower weight makes for a great warmup. This will get the blood flowing to your muscles.
Remember, don't use a lack of time as an excuse to miss out on a warmup. If you are pushed for time, simply cut out a set of each exercise - or reduce rest times - rather than skipping a warmup altogether.
Not planning a routine
Always try to plan out a routine thoroughly before you start. Otherwise, there's a good chance your workout will end up being fairly aimless and directionless. Have all the exercises, sets and reps all worked out and fixed in your head before you begin. To save any doubt or confusion, write it down or have it all saved into your smartphone.
Doing cardio before strength training
There are obviously clear benefits of mixing cardio exercise with strength training in the same workout. Cardio is crucial, but doing it at the wrong time can lessen the impact, have a detrimental effect and even lead to injury. Starting off with 30 minutes of jogging will increase your heart rate, but it's more than just a warmup. It will leave you fatigued and less likely to perform on the weights. It's better to warmup and then do the heavy lift work out of the way before hitting the cardio.
Not varying your routines
Sticking to the same routine is a sure-fire way to get bored. Crucially, as your body gets used to the exercise you do, you will see a plateau in terms of the results you get from it too. It's best to change a routine every few weeks - 3-4 weeks is the optimum.
There are countless ways of doing this: changing the exercises or by changing the other variables, such as reps, sets and recovery times.