1. Try spacing lines out as much as you can. Separate each line (or two or three, however many it takes to get you high) at least 45 minutes apart, and as the night goes on, try to take a break of at least an hour or so, time to get back to baseline, because after the initial rush, you're just chasing the high for the rest of the night. So yeah, spacing out your lines is, at least in my opinion, one of the keys to having a good night and conserving your stuff (so that that gram you just got 15 minutes ago isn't gone by the morning).
2. If you buy a reasonably large sum (like an eight ball or more), as you may have noticed, you'll be done with it a whole lot faster than you think. For example if you can make a gram last a whole week, then whenever you buy more that what you usually buy at a time (for me I usually buy grams at a time but occasionally an eight ball), take a spare bag and put like a gram or something in there an tell yourself that's all you got for the next few days, so that quarter ounce will actually last you the whole month instead of 2 weeks.
3. If you mix it with benzos (Xanax for me most of the time), and then tell yourself "Yeah, I'll just do that one last line then pop some benzos and go to sleep." then actually do go to sleep after.
Many time it's happened that I start out with a line and then pop 1 mg Xanax thinking by the time the blow wears off the benzos will kick in and I'll be able to sleep, but then I decide to do another line, and pop some more Xanax, then I reach the point where I've popped 4 mg and I'm feeling tired as hell but I keep on doing lines until I notice the sun is up and then just pass out. Then in the morning I wonder how I'm missing some bars and how that gram I had just bought was pretty much gone.
So basically, benzos make you do stupid stuff, so if you decide to pop some to help you sleep after the comedown, wait until after the coke starts wearing off to take them, get into bed, and resist any urge to do anymore.
And always try to save the benzos for after you're completely done railing lines for the night, unless you're really that anxious / paranoid that you can't stand it anymore, then I guess it's all right to pop a small amount of benzos (like in my case, no more than 1 mg alprazolam, then I usually save 1 or 2 mg for when I've put away the shit for the night and I'm ready to go to sleep.
4. Now, I might be wrong on this, but I've noticed the only true (and simplest) way to check if your blow is good is to simply just do a line. No amount of tasting, visually examining, or smelling is going to tell you how good it is. Just do a line or two, and if you're not feeling talkative and confident within the next few minutes, you probably didn't get good stuff. Of course this isn't the most accurate way to test the quality of your gear, but it's the simplest, unless you want to go into purifying it then measuring out the difference between the pure and the cut blow after.
Also, when you get it, try doing lines both from the top of the bag, and from the bottom of the bag too. A bunch of times I've gotten a bag that seemed really nice for the first half gram or so, very talkative, confident, euphoric and all, like a good coke high, then towards the second half of the bag it doesn't seem as good anymore. Of course, depends on your area and your dealer, but it seems most dealers put the cut towards the bottom of the bag, like that if you want to sample a line before buying you'll get the great coke high and think you got top quality shit, then later on after the first few lines, you'll notice it's totally cut shit
And also, rocky doesn't always equal good. I usually get chunky soft stuff with some powder in it and it's great, then another day I'd get it all rocked up and think "Sweet all rocked up, must be right off the key!" but then I notice how hard it is to break up and how there isn't a hint of shine on the rock. Rocky and hard = usually a sign of rerocked cut shit. You want chunky but soft and easy to break up (especially if you live in a humid area like I do, it gets all moist and sticky).
Also, YMMV, I'm just speaking from pure experience, for example, some people don't get talkative like I and some other people do, so don't take it for a fact that just because you don't feel like talking to someone you got bad stuff. I have a couple of friends, who I've done the same amount of blow from the same bag, and while I'm rambling my ass off, they're just sitting there, not really saying anything (at least not talking much more than usual).
All right, I'll stop with my coked up rambling now. I have a bunch more tips coming to my mind that I could write another fucking essay on, but they're probably not that helpful and maybe vary too much from person to person.