I've been trading full time since 1998. In the beginning obviously I was losing more than winning, until I read an article by Art Cashin. In it he wrote that when he wants to buy a stock, he hits the offer, and when he wants to sell it he hits the bid. That simple advice changed my trading forever. While I will often sit on the bid or offer these days, it's still the basic hitting the offer to get in or the bid to get out or shorting that gives me the most profit.