For example, if the margin is 50%, it means you have to have at least cash at 50% of the value of the shorted stock. So, in your account, you have 150% of the value you borrowed in cash. If the price of the stock goes up, you need more cash to keep your position, or your position will be closed out at your cost.
In order to protect your investment, there is tons of strategies and tools you can use, like options, futures, and other exotic derivatives. You can also use these vehicles to speculate, but it is very risky. Only those professionals can actually manage that. You might be lucky sometimes, but at last, you will have nothing if you have no relevant risk management.

Selling a stock short is riskier because our emotions often get us in trouble. It is a natural instinct to become most bearish and want to sell short just when the market is reaching an oversold-bottom. Making it even worse, we know that a stock can in theory go up infinitely. This means there is no limit to how much we could lose if the stock goes up and up. A third problem is that short selling takes patience. Mostly stocks go up or sidewise. Declines tend to be compressed into a relatively short period of time.

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