I will not lie... meth has been directly linked to some of the best sex I've ever had in my life. That being said, here's the rest of my two cents (as someone who has dabbled occasionally - i.e., a handful of times per year):
- Start out by doing A VERY SMALL AMOUNT, no more than a bump or a very small line. If it's quality meth it will be very potent, and you will not need to do a lot to get high.
- If you don't weigh much, do even less. Don't try to keep up with other people who do it frequently and/or weigh more than you (as was often my mistake). After the fourth or fifth day of being up continuously, you will regret it.
- After your initial high, do not expect to get much higher by continuing to do larger and larger quantities of meth. This will only result in you staying up for a much greater period of time before you crash.
- Make sure you eat something beforehand, even if you aren't hungry. You will not have the desire to eat for a very long time afterward!
- Drink water or other liquids throughout the duration of your high, because it's easy to forget to stay hydrated as well. Things like smoothies or milkshakes are a godsend because it's hell of a lot easier to drink your calories than eat them.
- Avoid doing meth with any other drugs your first few times (if at all), since the effects can be more than a little overwhelming. (I can vouch for this personally, as one of my first meth experiences involved snorting a couple oxys after a couple bumps of meth).
- If you're going to go out and socialize at any point in time while you're tweaking, take extra care to mind your physical appearance. If you've been up for days (whether or not it's due to drug use or not), people CAN tell - and more often than not they will say something.
- Finally, as a word of caution, do NOT start using meth frequently and justify it to yourself by saying, "It's a functional drug, I am using it to keep myself awake so I can accomplish task [a],
, [c], etc." My fiancee used to have a job in nightlife that required him to work all kinds of crazy hours, and rationalized his meth use in this way before it SEVERELY backfired on him a couple weeks later. Just be careful.
- And finally, good luck and have fun! 
J.