Investing is very different to trading. Selling and buying long-term investments is not generally considered a good idea - the costs of buying and selling are high, taxes come into effect when selling, and timing the market is near impossible. If, like me, you also want to take advantage of the short-term movements in the market, better to open a separate trading account for that purpose and keep your long-term investments as buy-and-hold. As the age old long-term investment advice goes: "It's not about timing the market, it's about time in the market."
My strategy to maximize gains from shorter-term movements (in my long-term investments) is to plan at the beginning of the year what I'm going to invest for the year. Then, each month, I buy what is cheap and then just hold forever. So, if the rand is strong, I buy my global ETFs, so that when the Rand weakens, I get further gains from the exchange rate. When the Rand is weak, I buy my local ETFs. Similarly, I buy my ETFs when they are at a low. But I never sell!!! For example, I bought all my 2019 PTXTENs already since it is at a 52 week low. NB: Note, however, that this strategy works for ETFs that are intended for long term investment, where the ETF is diversified. It does not work for single stocks, since a 52-week low in stocks may indicate weak financials or other reason. Buying the low in the long term is only really suitable for ETFs or unit trusts, not for single stocks!!!