All About Stock Slippage    

You can choose what you want to control when you place an optino - the price or the time. A limit order lets you control the price, with no assurance of a fill. When you place a market order, a fill is assured, but not the price. A calm and patient trader prefers to use limite orders, since those who use market orders keep losing silvers of capital in slippage.

Slippage tends to be a much bigger expense than commissions.

There are day-trading firms promising to teach traders to take advantage of slippage by trading inside the bid-ask spreads. Their techology does not guarantee success, while commissions from active trading negate any advantage.

Options strategy