While some methods of training are tried and true, the truth is that not every training method works the same for everyone. Many people seek a certain physique for themselves, but sometimes their body just does not respond to the common types of training.
The most common training systems include progressive overload, which is when every week you add more weight, while also trying to get the same number of reps as the last training session). There is also high intensity training, where there are less sets but performed with higher intensity (time under tension).
There are static holds, slower rep speeds, and other HIT principles (such as rest-pause training, where you rest briefly for only 10-15 seconds during a set, then try for a few more reps using the same weight), and deload weeks (which is a week of training with half the volume and weight, which gives your body a deeper recovery period while still training).
There are power lifting techniques such as dynamic lifting days, speed days, the tonnage system (which is more overall weight lifted through increased number of sets and less reps—overall there are more heavy sets lifted during the session), and many more techniques which I won’t even begin to list.

Finish Heavy, Start Light
Generally, most people are advised to start with heavy weights then go to lighter weight for the last set (or two). So, this may come as a surprise, but some people have better results following the opposite advice: they start with light/moderate weights for accessory work (with higher reps) then transition to their heaviest weights for the last sets (with only a few reps, try to aim for 5).
Essentially, do your workout with light/moderate weight which you can control and achieve maximum muscle contraction, then overload at the end. Reversing this order may lead to an increased soreness the next day.

Envision a Marathon
When you are riding uphill on a bike, there are many times when you want to stop, but if you are physically capable to keep going, you keep going. There are some people who swear by this mentality while performing difficult sets, to envision that they are on a marathon halfway through the uphill. Instead of thinking of sets as a number of reps, think of it as a bike ride uphill until you reach the top.
They say that this helps because sometimes when you only have a rep count in mind, you are only training yourself to fail or stop at a certain rep. They go beyond this and break the rules; losing count of reps and get their mind only focused on pumping the motion, repeatedly, just like peddling up a hill.

Max Reps During One Song
This technique works best with bodyweight exercises and accessory moves. Pick a song that really pumps you up. Now the goal here is to pump out as many reps as you can throughout the whole song until its finished. Lose your rep count and focus on the rhythm.
It’s okay to rest briefly when needed, but try to keep it to a 10 second rest period before you get back to pumping out more reps. Do as many reps (without counting!) as you can until the song is over.
Change up your stance if you need to. For example, if you are doing bicep curls, switch to a hammer curl until you need a break. If you are doing bodyweight squats, switch to a sumo squat for a bit. If you are doing pushups. Change your hand positioning, do partials, or whatever you need to do if that’s what it takes to pump out more push-ups. If you plan to lift heavy during this, you will likely have form failure before muscle failure, so do bodyweight when you can, otherwise keep it light/moderate.
By simply changing your mind to focus on the rhythm of the song, you will go beyond a normal failure point. Your muscles will have to work under both endurance and fatigue. If you are physically capable, keep going until the end of the song, even if you start to get exhausted after a minute or two. The next time you train that muscle group, guaranteed you will be stronger. And definitely expect to be extremely sore after this.

Think Beyond Ab Work
Look around the gym, or even the general public. You never see a fat person doing hundreds of reps for abs. The people with highly developed abs usually do hundreds or more reps of abs (just look at Serge Nubret), otherwise they are likely just blessed by the genetic gods. You should try out doing a high-rep ab routine for overall health and conditioning (not just to work your abs).
Doing higher reps of abs will help you get into the habit of contracting your ab muscles harder and more often throughout the day, as well as with all other exercises (having an engaged core during non-ab exercises is very important). To keep a streamlined V-taper and a tight waist, for the most part weighted ab exercises aren’t necessary.
Some people try to work their way up into doing hundreds (or thousands) or reps for ab exercises. To do this, pick any combination of ab exercises and work on a rep goal. Essentially, make up your own ab circuit. This takes away the monotony of training abs (nobody wants to sit and do 1000 sit ups). You can do crunches, leg raises, heel touches, plank dips, bicycle crunches, there are hundreds of variations of ab exercises you can do, so be creative.

Wider Weight Range as You Develop
Closely follow any fitness bro and you might notice that some days they’re lifting insanely heavy, and some days more moderate, a huge gap with their weight range. This is not because they became weak, but because the more advanced you become, it is easier to lift based on how you feel.
Within the first 5 consistent years of weight training, you generally gain the bulk of your size. After a certain point, its not always about how heavy you can go, but more about the muscle/mind connection that you establish.
If you focus hard enough, you can make a measly 10lb dumbbell feel like 50lbs. It is possible to be challenged if you develop a muscle/mind connection through focusing on the contraction of the muscle. It may take a lot of reps and experience to develop this, but with this method you can still see results. Just because you can lift jaw-dropping amounts of weight, doesn’t mean you always have to. It’s great to have the ability to, though!
Keep an open mind when it comes to taking care of your body and developing it. It’s not always supplements, drugs, or gorging yourself to get enough protein. Be smart and open-minded and remember that you never get too old to stop learning.