I have seen countless athletes and gym rats alike who plod through their training sessions at such a slow pace, with huge rest times, despite complaining they do not have much time to train. When they walk through the gym doors, they are making excuses and saying they will probably be cutting it a bit short, or just doing a bit of this, or a bit of that, or "going light", regardless of the fact that they did this the week before and the week before that. I have seen people amble through warm ups, performing a token gesture towards joint mobilisation and SMR (a token gesture i suppose is better than nothing at all). They idle through a few warm up sets and then somehow out of nowhere expect to hit personal bests and break records all of a sudden when they finally get to a weight which is challenging. If you don't turn up with the right attitude, and don't know exactly what you are trying to achieve, it will be by sheer luck and coincidence if you make much progress.
It is essential to have a plan and stick to it. Know before you turn up to train what you want to get out of your session. Remember how previous training sessions have gone, establish which areas need work, and focus your attention on the task in hand. I fully understand that a lot of people come straight from work, and their mind is likely to be elsewhere, but I feel it is essential to get your head right before you train. If you come through the doors with your mind elsewhere on other things, it is unlikely you will be able to focus once in the gym. I would recommend that those wanting real results, take just a couple of minutes before they come into the gym to focus their mind, think about what they are trying to achieve, and make sure they know exactly what they are going to do. Just sitting in your car for 3 mins is enough to do this. Its enough time to clear your head of everything else. If you've had an argument with your other half, or if your boss has given you grief, or if you are worried about this or that, whatever it is cannot be on your mind when you start to train.
If you want to lift big weights it takes a huge amount of concentration and with that a whole load of determination.
Find a pre-training ritual, a way of preparing yourself before you train. It might be 15 mins long and include taking any pre-workout drinks, applying tiger balm or deep heat to sore joints, or whatever other ritual. Or it might be as simple as sitting in your car with the radio off and just taking a few seconds and a few deep breaths. Either way it is vital, in my opinion, to ensure that you don't come and 'drift' through the whole session not really being switched on. For those of you who play sports, in particular contact sports, most of you will have pre-match rituals which you rely on to get you feeling prepared to compete. A similar thing should be in place for when you train. Especially if you are looking to maximise the results of your training.
If you are happy having half arsed training sessions where you finish up with that thought in the back of your mind that you could have done more and weren't really focussed, then carry on how you are. Personally I am certain I train more effectively when I start the session right.