So my first undergrad job (in the campus dining hall) gave me my first discretionary income, and I didn't have much discretion in spending it. I made all the important payments for tuition and textbooks and everything, but I didn't bother saving beyond that. My advice really boils down to this: Save as much money as you can (while you can), and stop wanting stuff. But I have a few more specific suggestions that I think would have come in handy when I was younger.
1) It's a trap! If you get a credit card (and these days, almost everyone needs a credit card, if only to build good credit for later in life), only spend money that you already have. I usually factored in my incoming paychecks and impending bills and all that, so I could be sure to pay my balance in full and on time, but after a while I stopped keeping track of it — and once you do that, it's easier to dig yourself an even deeper hole of debt. Do not carry a balance on your credit card, particularly if you have a high interest rate, and remember that cards often start out at low interest rates and then get significantly higher later.
2) Avoid eBay. This was a huge distraction and money waster for me, as I made up for a childhood without lots of things by buying all of them from eBay. It might seem like a bargain to get that Darkwing Duck action figure you always wanted for only $3, but all those little purchases add up. So whatever your poison is, whether it's Etsy, Think Geek, or eBay, stay away!
3) Money doesn't buy happiness. It seemed that the more I worked and the more money I had, the more I spent, as if the value of all that time could only be measured in stuff, or fine dining, or going to see movies and shows. I think it's important to work, and I feel like most of us have to, even authors. I actually recommend that everyone work in some kind of service industry at least once in a life (my first summer job was as a doorman), so that you can learn the value of being kind to others, as well as how much money is really worth. Weighing the cost of a shiny new DVD player against the number of hours it took you to earn it is a quick way to determine whether you really need a new DVD player. Now this might seem contradictory, but I think it's even more important to get a job you enjoy, that pays you what you need. I had a great paying job as a technical writer, but I didn't like working for a corporation; when I switched jobs and halved my salary, but had more fun at work and time to write, I was much happier. Money doesn't buy happiness, but it can buy you security — especially if you save.
I still struggle to follow all my own advice; it always seems like there are more unavoidable expenses the older I get, which makes it even more important to limit the avoidable ones. And sometimes you do consider the cost analysis of debt versus things like a once-in-a-lifetime trip. (Pro tip: Once you're in damage control on debt, zero percent interest cards are your friend.) As I prioritized writing in my life, I had less time for other pursuits, so I stopped buying as many games, movies, and so on. (I still buy lots of books, of course. I'll always buy lots of books.) Using Netflix and libraries also helps cut down on expenses and clutter, because space in my apartment is just as finite as my funds.
So where do you spend most of your money? What poses the biggest ongoing threat to your allowance or paycheck?