Realize the magic of compounding interest and put that realization into practice. Start investing early and regularly. Be consistent. At the beginning of each month, put aside a sum of money and invest it. Go for low-cost index funds. Arrange for the sum to be transferred automatically. Then forget about it. In 40 years, you will have a large sum of money stashed away.
"Never catch a falling knife"!
Means you should never ever buy more of an asset just because it has dipped. Of course it could work out well, but it may as well be that you end up just having more of it and it still is down. By doing so, you just gamble that the price will go up again, but gambling is not investing.