Play the long wait cards at the correct moment. In longer battles, play a few quick cards first then use the long wait cards to make sure that they are not killed before you get a chance to play them. As soon as you have 100 gold, spend it on a card. It may be better than what you have, and if not, it will gives you extra upgrade points. Some cards can boost the stats of other cards, but only if they are of a specific race or class. You can determine a card's class by looking at the upper left corner, and the race from the card's color. Make sure to play them at the same time or you will waste a special skill.
