1. Don't borrow money for a useless degree. Save money first, then go to school. Or get the military or an employer to pay for tuition.
2. Don't borrow money for a car you don't need. Reliable used cars can be had for $2500 or less all day. My last airport junker cost me $500 to purchase, I put 200k miles on it, and sold it for $500. Average cost of maintenance per year was $1200. Or get a bike. Or take public transport.
3. Don't rent a luxury 1BR apartment for $1500. Rent a non-luxury 2 BR/1BA apartment for $1200 and split it with a friend.
4. Don't borrow money for Starbucks, drinks out, and whatever else that millennials use credit cards to blow crazy money that they don't have.
5. Don't adopt a furry "friend" so unnecessarily early in life. Some study I found on the internet says that 57 percent of millennial households own a pet and the generation is 77 percent more likely than other generations to get a pet before they marry or have children. And many millennials make their home buying decisions based upon their pet. Something like one-third of millennials, which is the largest group of home buyers, buy a new home to have more space or a better yard for their dog. This age group also spends more on their dogs, with purchases of pet care items, toys, treats, and vet visits. None of this is necessary, but they won't admit that when they're whining about being unable to save for a house or a retirement when they're spending at least $15,000 on their dog over its lifetime (my guess is that it's much higher than that number. Save the dog for the retirement when you need an excuse to get out of the house and go to the park, enjoy not having that responsibility when you're 25-35 so you can go pick up extra shifts or spend a weekend out of town.

There's a ton of other ways to save money without living a miserable life, however millennials just can't admit that they're suffering from extreme lifestyle creep that is a product of being raised by boomers who've benefited more from economic growth than any other generation in the history in the world. Millennials are trying to live the boomer lifestyles that their parents do, without being willing to live well within their means for a decade or two to establish themselves financially first.