Of course the best way to prevent margins is to follow a big strategy for trading. Fair money management is the key to good strategy. And this money management will protect us in such adverse cases while the market moves in opposite direction. And we have to lock the position after a certain position !! and the only thing I know how to prevent margin calls is to use a small lot size, and always use stop loss on our trading account. We cannot accurately predict the possibility of the next market direction even if we try harder. Stop loss is our only weapon to last longer in forex.