TRUCKEE, Calif. - Will Rogers, noted American humorist, once said, "Don't gamble; take all of your savings and buy some good stock and hold it till it goes up, then sell it. If it don't go up, don't buy it."Now there's some great advice, and so simple - "if it don't go up don't buy it" - all investors have to do is not buy any stocks that don't go up. Seriously, what makes a stock go up? In simple terms, it all boils down to cash flow. When you buy a stock, you have a claim on future cash flows. If you buy stock in a company that has strong cash flows in the future, you'll probably do pretty well. So how can you project future cash flows?One way is to look at where demand will be in the future. It can be very difficult to predict trends in fashion and consumer products, but some areas are a little simpler. There are supposed to be about 9 billion people on this planet by 2050. Demands for staples like food and energy should be very strong.Changes in the way we consume energy are inevitable. Fossil fuels are a finite resource that will be depleted at some point. Oil prices, and subsequently, gasoline prices, can be volatile and subject to rapid price spikes due to unforeseen geopolitical events like unrest in the Middle East. Oil could spike and remain elevated for an extended period.Natural gas usage should increase, it is very clean burning and the US has a supply that could last about 200 years. The main issue with natural gas right now is in the extraction process, known as hydraulic fracturing, or "fracking." The fluids used in the fracking process can pollute groundwater.According to a recent article in Barron's, China is projected to use four times as much natural gas by 2035 as it did in 2011. The country is having serious problems with air pollution right now. Japan and India are also shopping for new supplies of natural gas. Natural gas producers in Australia are especially well positioned to meet the growing demand from Asia. The US has the potential to become an exporter of natural gas, but we are behind the Australians by several years in developing the fields and the export facilities.Investors today can easily add energy to their long-term investment portfolio by using ETFs that are based on gasoline, oil or natural gas and by looking at the producers of those products. Remember - try not to buy the ones that "don't go up," like Will Rogers said.Kenneth Roberts is a Truckee based Registered Investment Advisor. Information on his money management service can be found at his blog at www.sellacalloption.com or by calling 775-657-8065. Past performance does not guarantee future results. Consult your financial adviser before purchasing any security.