Complete lots of sidequests and be greedy when the person who gave you the quest wants to give you a reward by persuading him/her for something extra or just threaten the person. Steal, loot and sell that stuff.
Pick up any piece of armor and weapon you can, even if you wont use it. Heavy plate armor sells for multiple gold pieces depending on what its made of as well as weapons. Also, make potent lyrium potions, making a potent lyrium potion actually costs less than its sell cost, so you can make infinite gold by making and selling them
You lose nothing by asking for a reward. Always ask for money and persuade for the highest amount.