In general, your thinking speed can be improved by training. In your particular world, for example, casting spells competitively or forcing your brain to think tactically(not strategically) by playing games or solving math problems, for example, could be a reliable way to sharpen your brain and get it used to thinking quick. Some people are naturally going to be better at that than others, but save for the rarest of cases, sheer effort usually trumps sheer natural talent.
Mostly by changing your diet and your lifestyle, you can become more or less energetic. I've recently stopped eating sugar, and, on top of losing almost 40 pounds, I also find that I can get through the day with a lot more disposition to do stuff that needs doing in general, instead of just when I eat. On this note, however, sugar does make you more energetic for a while, so it could be a viable tactic to ingest a big quantity of sugar to temporarily greatly improve your agility. The cost of this, however, is that when you experience the sugar crash, your agility would be severely hampered for a couple of hours. In a life or death scenario, maybe it's worth it, who knows?
Reflexes are involuntary, instant movements, such as retracting your hand without thinking when you touch something hot, or when the doctor taps your knee and your leg moves, that are "hard-coded" into your muscles and are not processed by the brain at all. Thus (Or at least as far as I am aware) there is no general training that improves all of your reflexes at the same time. What can be improved with training, however, and what I suspect you meant in the first place, is reaction time. There are several ways to improve reaction time, such as:
- Repeating the same motion over and over(The motion you want to get better at): This builds muscular memory, which is (sort of) an user-generated reflex. Your muscles get used to performing a certain set of motions in a certain order, and after a while, instead of having to give each mental command to your hands with intent, just giving the first one in the chain is enough to trigger all the other ones. This training is only good for the thing you're training and may even be detrimental to other motions - just ask any musician that made a mistake when playing because their fingers moved sort of automatically, but to the wrong chord. Practice doesn't make perfect. Practice makes permanent.
- Not drinking alcohol: On top of hampering your decision-making skills, alcohol has been shown to severely increase your reaction time. Don't chug and cast, kids
- Ingesting water frequently: Even a few hours without water can cause a significant increase in reaction time, too.. Remember: Every good mage is a water mage
- Runnning on uneven ground: This might sound weird, but running on uneven ground forces your brain and body to adapt to all sorts of stimuli, quite literally, on the run, so your reaction time, especially related to movement, increases. If you want to live thinking on your feet, you better make sure that your feet are good at thinking, no?
- Being phyisically fit in general: As with everything related to your body, having a healthier body leads to more efficient processess. Here's a piece of wisdom: You'll never cast a spell in 1 second if you can't run for 90
I think that's about it. You might find that of the 3 things you've described this far, this one is the hardest to improve. You could even make this one be the thing that sets the mages that are simply competent from the ones that are truly masters of their craft. Hope this helps!