The average human attention span is getting shorter. In 2000 the average human had an attention span of around 12 seconds and in 2015 this was 8.25 seconds. Hope is not lost because the ability to concentrate is one that can be practices and improved.
Sleep
A good night sleep is essential for being able to concentrate. Sleep has been shown to improve cognitive performance, physical performance, hormone output, and many other things including immune system function. There is simply no tool to overcome sleep deprivation and remain concentrated. So if during more than 20% of your nights your sleep quality isn't good, then start with improving your sleep. Nobody's sleep is perfect but during 80% of your nights you should get high quality sleep.
Defocusing
Think about dep focus as a workout. Between every set during resistence training, you take rests. The same should be done in between focus sessions. This will increase the quality of your next set. During such a rest period you can do tasks that don't require a lot of concentration.
Also note that a well trained person, regarding concentration, can do around 3 sets of 90 minutes of focus work per day. That doesn't mean the person only works 4,5 hours per day but tasks that require less focus can be done in between each focus session.
Don't eat too much
Recognize the relaxed feeling after you've eaten too much? Then your body is in the so-called parasympathethic mode. During this relaxed state it is harder to focus and thus should be avoided when you want to concentrate. Make sure you are well hydrated and have enough electrolytes, otherwise you still won't have enough energy.
Listen to 40Hz Binaural Sounds or White/Pink/Brown noise
A number of peer reviewed studies have found that 40 Hz Binaural Sounds improve concentration. No studies have shown that White/Pink/Brown noise improves concentration, but it does improve people's ability to transition into a concentrated state. These sounds/songs can be played the 5 minutes before doing a focus session, or during the whole session.
Cold water exposure
Your stress levels increase when exposed to cold water. This stress then releases adrenaline which changes our visual field to a more tunnel-like focus. We tent to be stressed about a specific thing; we start wondering and thinking about what's going to happen next and what led up to this stressful situation. Deliberate cold exposure increases adrenaline and dopamine levels in our brain and blood which is good for our ability to concentrate. These increase levels of adrenaline and dopamine can last for several hours.
The water should be uncomfortable to the point that you want to get out, but you can safely stay in for one to five minutes.
Meditation
Set a timer for 13 minutes, sit or lie down and close your eyes, and focus on your breathing during these 13 minutes. Unless you are a very experienced meditator, your focus is going to drift away every 5, 10 or 20 seconds. Important is to refocus to your breathing the moment you realise you drift away. This drifting and refocusing is actually important part of training your concentration. If you wouldn't drift, then doing this meditation wouldn't be helpful for improving focus. Studies have found that doing this practice not only improves your focus, but improvements in mood, sleep, memory, and reduction in stress have also been shown.
Source: Focus Toolkit: Tools to Improve your Focus & Concentration by Dr. Andrew Huberman