In this case, you can place a limit-buy order a few pips above that support level so that your long order will be filled when the market moves down to that specified price or lower.
The best way to protect yourself is to use limit orders when placing an order to buy or sell a stock or an exchange-traded fund. (See How Your Buy and Sell Orders Get Filled for more.) Look at the bid-ask spread and set your price limit at the bid for selling a stock and at the ask for buying a stock.