Instructions
Sell items you don't need. Visit vendors when you are in towns and sell all of your low-level armor, weapons, and trade items.
Make and sell traps. If you have the trap-making ability, you can sell traps in sets of three to Allison in Lothering for 50 silver. This repeatable quest becomes unavailable in later stages of the game.
Complete side quests. There are numerous optional quests offering gold as a reward for completion.
Craft Potent Lyrium Potions. If you have the potion-making ability, these potions can be sold to vendors for a significant profit. All crafting materials for the potions are all available from vendors for much less than the cost of the finished product.