1. Don't eat until you are really full. Eating until you're really full takes a lot of energy to digest, and you're eating to gain energy, right? Only eat until you're not hungry any more.
2. Meat hinders the flow of certain energies. Specifically it will hinder the flow of most magickal energies, and if you eat meat until you are full it will hinder all energies badly. While it won't otherwise bother your energy flow, a more vegetarian diet can actually help not only energy flow, but your ability to see into the spirit realm. If you're having trouble with AP or OoBE, it might be that the meat in your diet is holding you back.
3. Alcohol is not only going to disrupt the steady flow of energy through you, but it's a very yin substance. This means it will make your energy sluggish and lacking in force. As a general rule, if it's a small enough amount to be of physical good (a bottled beer every now and again is good for the brain) then you're not doing much to your energy other than making it a little slow while your body digests and processes the drink, but anything more than that and you're unbalancing your ki.
4. One of the sources of ki your body has is food. Processed and altered foods will give you processed and altered ki, which is sometimes worse than getting no ki at all. The more organic, the better.
1. Know your limits and don't ignore them!! There's nothing wrong at all if you can't use for very long when you start out, just like there's nothing wrong if you can't bench-press your body weight the first time you work out. You need to work up to that point. Pushing your limits is good, but just rushing past them without caution is going to do nothing but cause a burn out, which will not only stop you from training for a while, but stop you long enough that when you can resume practice you will most probably be in worse condition than when you started.
2. You have a finite amount of ki. Yes, you can draw in more when you're running out, but just like there being a limit to how many times you can recharge a battery before it burns out, there is a limit to how many times you can fully replenish your ki before you need to take a break. Taking a break won't slow your progress, even if you need to stop for the rest of the day. In the end, this rest period will not only allow your body to "grow" new reserves of ki, but they will be bigger, stronger, and more healthy.
3. Have a work out plan. The bigger the tech, the more energy it will use up and the harder it is on your system. Do a few "warm up" techs, do any big techs you're trying to learn or perfect, and then spend the rest of your time working on stuff you can perform but want to be better at. The warm up should be longer than the time spent on big techs (but not by a huge amount), and the "cool down" period where you work on techs you can already perform should be around as long as both put together, going as long as 1.5 times as long as both. So if you want to work out for an hour, try this:
15 Minutes of warm-up techs (like Ki balls and Ki shields)
10 Minutes of hard training (Genki Dama)
35 Minutes of cool-down period (like Ki blasts)