It's not how much you make, it's how much you spend
This is about both living within your means, and finding happiness in what you already have. Starting with the first, it is really important to have a budget for your spending. You need to have a good idea of the amount of money you will make in a certain period of time, and then organize how much you plan to spend on various things over that period. Start with the fixed spending – the stuff that is consistent every month (rent, utility bills like electricity and cell phones, groceries, etc.). Then look at the stuff that doesn’t happen every month, but you know is coming; stuff like doctor bills, replacing a broken phone/computer, etc. You have to save money every month for those things, even though you won’t spend it every month (maybe even put that money in a separate account so you don’t forget). Lastly, everything you have left is considered “discretionary spending money”. This is for stuff like going out to dinner, travel, clothes, etc. At the end of the process, you should have a good idea of what you can spend each week on stuff you want to do, and how much you need to be saving for the other things you want/need. I think the book “Blue Chip Kids” has a good section on this stuff (and is a good book for basics on a lot of different money-related topics). There are also several websites about living frugally (www.mrmoneymustache.com is one that has had some interesting articles, but there are many others).
The other side of this quote is the idea that you shouldn’t always be chasing after “more”. You don’t need a ton of money to be happy, and often just the opposite is true. Money can lead to pride and showiness, which leads to frivolous spending (fancy cars, clothes, etc.). Find something you love to do and be great at it, but don't just chase after promotions and money, which just leads to unhappiness. Be happy with what you have, however modest, and make the most of it. There is a related saying, “It’s not getting what you want, it’s wanting what you’ve got”.
Now, there is nothing wrong with treating yourself to something nice now again, but make sure you can do it comfortably within your budget; you may have to save up for years to get/do it. Think of how much fun we had on the RV trip, and that was not luxury living. Don’t confuse wealth or materialism with success, they are not often related.
Two additional related thoughts:
Be very wary of credit card debt. Pay your bills in full every single month, on time. Carrying a balance on credit cards is crazy expensive and just not ever worth it. Most of them charge ~18-20% interest, which means that if you have $1,000 in credit card debt, you have to pay $180-$200 to the bank every year on top of the $1,000 you owe. That’s just stupid.
Try to save 20% of everything you make. Put it in an account that you never touch. You will be amazed at how it grows as you add more and as it earns interest over time.