Luckily for you, my advice is free. Without further ado, here are the easiest ways to update your wardrobe for the chillier parts of fall ... sans the wallet emptying.
I'll start from the bottom.
Footwear
Your shoe solution can be as simple as you want it to be. Flip-flops are almost always acceptable. They might get some pitying or confused looks once the snow begins to fall, but a stylish guy goes out with his toes out - at least until November.
Other than flip-flops, the shoe game is not too complicated. Unless your sneakers have wheels or light up as you walk, you should be good to go. Shoes are notorious budget-denters: They represent one part of everyday style that is neither cheap nor easy, so you will likely be forgiven for sticking to sneaks. Once the snow comes, however, the whole story changes. You'll have to check back later for guidance on that.
Pants
Like flip-flops, jeans are "in" nearly all the time. Unless your jeans are late-'90s baggy or, for that matter, the same pair you wore in the late '90s, you should be fine. Durable and warm enough, jeans never really get you in style trouble as long as they are a simple cut - not too baggy, not too tight - and the right length, falling so that they cover some of your shoe but don't graze the floor.
For more formal situations, Princeton etiquette seems to mandate khakis. These can be more expensive than one might like, so thriftiness is potentially useful here. One must first, however, recognize the efficiency of the J. Crew sale section (JCSS). "Preppy" or not, you can't deny the convenience of the Palmer Square store. JCSS consistently has a decent enough selection to keep you (literally) covered. There are, of course, plenty of other stores in the area where one can buy all this, but JCSS is unavoidably convenient. Unless you live in Forbes. Remember, with your student ID you also get an additional 15 percent discount.
Shirts
T-shirts are inevitably abandoned when the cold weather hits, so you should try to get the most out of your T-shirts before then. As a Princeton student, you should have enough free shirts to sustain your fashion needs through the fall. Of course, if this were anywhere else, we would advise that you keep the orange to yourself. Here, though, the garish orange simply serves to announce your fashion know-how. After all, you'll be asserting your individuality and quirkiness ("Wow, orange!") in a context that completely accepts your fluorescent threads.
Though they are less popular, long-sleeved T-shirts also come in handy this time of year. Since you'll often be wearing a sweater or jacket anyway, these need not be too fancy. JCSS could come in handy here, too, but if you want to spend even less, befriend someone with a car who knows how to get to Wal-Mart.
Sweaters

A good sweater is easier to come by than a bad one, but there are still duds out there. Thus, I advise that you use the following categories of evaluation to determine whether a prospective sweater is worth it.
Softness: Does the sweater make you want to stay in it, or is it pricklier than a senior talking about grade deflation? If it's the latter, toss it.Neck: Basically, any kind of sweater neck except for turtleneck is acceptable. I'm not even sure turtleneck sweaters exist for men, but this is a fair warning.Style: This may seem like a particularly subjective category, but there's a pretty easy way of telling whether your sweater is wearable. No teddy bears, unicorns, snowflakes, Christmas trees or other stitching that make one yearn for the "good old days." You're in college now. Stop it.
Unfortunately, a good sweater is rarely inexpensive. This is where you should consider abandoning the JCSS and other in-town stores for the U-Store. It's unlikely you'll find a great sweater, but a decent sweatshirt is another story. Anyhow, it seems there is little Princeton students enjoy more than announcing their school through clothing.
Extras
The fall can get cold pretty quickly, so it's always good to be prepared. Get a scarf and a wool hat from JCSS or the U-Store. These tend not to be too expensive but are fairly necessary unless you're one of Princeton's handful of Alaskans. Scarves are self-explanatory and hard to mess up: Stripes can be worn to sporty, hipster or prepster effect depending on the rest of the outfit.
Hats, however, are not quite as simple. If you want to look like the cool, quirky kid, go with earflaps: They're just goofy enough to detract from you looking completely ridiculous. Otherwise, your best bet is to go with a hat that doesn't have a pom-pom on top, or any other unnecessary adornments. That being said, if your main concern is budget, just find an old Santa hat and wear that everywhere. That way, you're sure to get noticed.