If you are looking for an improvement above and beyond the discussed stimulants, I would greatly recommend the CILTeP stack. I don't have the specifics on how it works, but from what I recall it uses increased dopamine levels to potentiate cAMP-4 levels (using forskolin) and some sort of bioflavanoid to benefit absorbtion of the forskolin. The stack is as follows:
100mg-150mg caffeine
L-Phenylalanine (not sure the exact dose, but the amount in a 5-hour energy works)
Artichoke Extract (500mg)
Forskholi Extract (20mg)
I haven't heard a bad thing about this, and everyone who uses it goes for months seeing zero decrease in its effectiveness. For most people, the Artichoke extract is enough, but some people prefer to use Quercetin which causes the effects to last much longer (the people in the first group report insomnia and so prefer to stick with the artichoke). Check online if you are looking to supplement.
Note that this stack is VERY POTENT. I am personally super-sensitive to stim-induced anxiety, but I feel zero anxiety on this stack, just extreme focus and motivation. The added benefit is that while most "focus drugs" tend to make me less social, this stack actually makes me MORE social, I find myself being extremely extroverted and talkative since I started taking this. I may be overhyping it, but I strongly endorse the stack.