Do your own nails - this is an obvious one. Manis are expensive. Even if you buy really expensive nail polish (which I recommend, because it won't chip as fast and make you mad), it's still going to be under $10 a bottle. Know how many manis you can get from one bottle of nail polish? I'm not sure exactly, but I know it's a lot. Essie and OPI are my favorite brands, although I've recently been introduced to Obsessive Compulsive Cosmetics (which are vegan and cruelty-free) by my dear friend Shauna, and I'm HOOKED! I have "Pansy" and "Blackboard" and I love 'em.
Use eyedrops regularly if you have red eyes - If you're a sleep-deprived mom, you probably have slightly red eyes all the time. This is the same principle as bleaching your teeth. Use eyedrops regularly to moisturize tired eyes and get the red out. If the whites of your eyes are truly white, you look healthier, happier, brighter, and better, even when you're not wearing makeup.
Bleach your teeth - Less obvious tip, because buying at-home bleach kits can be pricey. But IMO it pays huge dividends. Having white teeth makes your whole face look brighter, even when you're not wearing a drop of makeup. I also drink my coffee through a straw to minimize contact with my teeth, and try to choose white over red wine to avoid stains. I like Rembrandt Whitening Strips, which unfortunately appear to be out of stock. The two-hour whitening kits sound great, but left me with SUPER sensitive teeth (as did Crest Whitestrips). If you experience sensitivity after whitening, take some advil or tylenol and know that it'll only last for a day or two.
Anti-aging lotion - You don't have to spend a ton of money on anti-aging products. The drugstore brands are fine (I prefer cruelty and paraben-free). Actually I've heard that the drugstore brands have more money to do research and therefore have better products...but that's just a rumor. At any rate, my feeling is some kind of anti-aging product is better than none, no matter what the price. Also, if you're going to splurge on one anti-aging product, eye creams are what you should go for.
Sunscreen - When I was ten years old my mother gave me a bottle of Oil of Olay classic moisturizer with SPF 15 and told me to put it on my face every day. I listened. I firmly believe this is one of the reasons I don't look a day older than 25 (ahem-ha-ha-ha). I'm still on the hunt for a good non-greasy paraben and cruelty-free daily facial moisturizer (if you love yours please let me know what it is!), but I've used one every day since the talk with my mom fifteen (ahem-hahahahah) years ago. Sunscreen on your face is a MUST in every beauty routine, even in the winter, even on cloudy days, and it's never too late to start using it.
Pluck your eyebrows - Costs you nothing, but well-groomed eyebrows will frame your eyes and face and make you look neat and tidy even when you're not wearing makeup. There are tons of resources online that can help you get started. As a veteran plucker who started at age fourteen (you seriously don't want to see any photos of me pre-plucked, it was scary), my best advice is to periodically take a step back from the plucking and get the full-face view to see how the shaping is coming along. It's tempting to get stuck in the super-magnified mirror and over-pluck. If you're nervous to shape for yourself the first time, go to a professional once to get advice on the best shape for your face. Take a photo when they're done and use it as a reference in the future - all you have to do is maintenance.
Don't dye your hair - My hair has been artificial color-free for the past ten years (until I recently got bright purple lowlights - I'm a hip mama now!), and one of the big reasons was the cost of maintaining color. Every six weeks really adds up.
What are YOUR low and no cost beauty tips and tricks?