The most obvious way to increase demand is by adding weight. It’s smart to do this in small increments - remember PROGRESSIVE overload - to let the body adapt gradually. I suggest adding 2 to 5 lbs every time you feel like you could use more of a challenge. A good rule of thumb is if you can comfortably complete 2 extra reps on the last set, then you should consider increasing the weight. Remember to do this while also maintaining good form and feeling the target muscles work. Sometimes adding too much weight shifts the work into non-target muscle groups and this isn’t ideal. A good example of this is with Bulgarian Split Squats (BSS). In my experience loading these up too heavy will shift the work from the quads to the glutes and I just end up with really sore glutes - not ideal if I’m trying to target the quads. It’s all a balancing act and making sure your form isn’t sacrificed is priority number one. Quality over quantity!!!