Buy undervalued assets ("buy low, sell high"). If you're talking about stocks and other assets, you want to buy when the price is low and sell when the price is high. If you buy 100 shares of stock on January 1st for $5 per share, and you sell those same shares on December 31st for $7.25, you just made $225. That may seem a paltry sum, but when you're talking about buying and selling hundreds or even thousands of shares, it can really add up.
Invest for the long run. [9] Choosing good-quality investments can take time and effort. Not everyone can do the research and keep up with the dynamics of all the companies being considered. Many people instead employ a "buy and hold" approach of weathering the storms rather than attempting to predict and avoid market downturns. This approach works for most in the long term but requires patience and discipline. There are some, however, who choose to try their hand at being a day-trader, which involves holding stocks for a very short time (hours, even minutes). Doing so, however, does not often lead to success over the long term for the following reasons:
Mutual funds. A mutual fund is a basket that contains a bunch of different investments — often mostly stocks — that all have something in common, be it companies that together make up a market index (see the box for more about the joys of index funds), a particular asset class (bonds, international stocks) or a specific sector (companies in the energy industry, technology stocks). There are even mutual funds that invest solely in companies that adhere to certain ethical or environmental principles (aka socially responsible funds).
Buy companies that have little or no competition. Airlines, retailers and auto manufacturers are generally considered bad long-term investments, because they are in fiercely competitive industries. This is reflected by low profit margins in their income statements. In general, stay away from seasonal or trendy industries like retail and regulated industries like utilities and airlines, unless they have shown consistent earnings and revenue growth over a long period of time. Few have.
If people see that companies are doing more or less manufacturing, or hiring more or fewer people, that can influence the way people feel about the economy. If people think things are good, they tend to buy stock on the thought that companies are hiring (or doing more manufacturing, which leads to hiring because people are needed to make things) which gives people jobs and disposable income. People with disposable income buy more goods and services, which is good for company stocks.

A simplified look at a successful investment is one where an investor buys a company at X amount of dollars, holds on to the company for an extended period of time until its value has grown to the point that they feel comfortable selling it, and then sells it for a profit. If the company offers dividends, they may also accumulate profits along the way without having to sell any of their shares.
It’s important to consider transaction costs and fees when choosing your investments. Costs and fees can eat into your returns and reduce your gains. It is vital to know what costs you will be liable for when you purchase, hold, or sell stock. Common transaction costs for stocks include commissions, bid-ask spread, slippage, SEC Section 31 fees [31], and capital gains tax. For funds, costs may include management fees, sales loads, redemption fees, exchange fees, account fees, 12b-1 fees, and operating expenses. [32]