Once you leave the Heroes Guild and can start exploring go to Bowerstone South (with at least 1500 Gold and at least a trophy or 2).

There is a house near the school which you can purchase and rent out. Once you rent it out you can go to find a vacant bed (one that is near the exit of the building). In Bowerstone South there are shacks down the road from the school that has a bed on the floor right near the door to the shack.

The reason why you want a bed near a door is because when you sleep in another persons bed it will be considered trespassing and you will gain a bounty. But if you can sleep in the bed and exit the building before the screen turns black then you will gain no bounty and time will still pass.

When a day passes in game you will get a bag of gold for rent. You can gain up to 3 bags of gold per house you own. So if you sleep for 3 days ( 6 times in a bed because when you sleep it goes through half a day so sleeping twice will be considered a full day) you will see a bag spawn near your house's sign. The highest amount of bags each house can spawn is 3....so sleeping longer than that will just be a waste since you will not gain anymore rent until you pick up the 3 bags.

With the money you gain you can upgrade each house up to 3 times (I believe) giving the house more value thus giving you more rent. You may also place trophies you earned from quest on the walls of the house for display thus increases the value of the house even more. (the better the trophies the more gold)

Once you fully upgraded the house and have placed a few trophies in the Bowerstone South house you can almost gain 1000G in 3 days (which takes maybe less than a minute to gain).

Do this as much as you want and use the gold to buy new gear or items.

Each town has a house you can purchase, upgrade and rent out for gold.

Bowerstone South House can be bought for 1500G
Oakvale House can be bought for 5000G
Knothole Glade house can be bought for 9000G
Hook Coast House can be bought for 29000G
Snowspire Village House can be bought for 25000G
(Each towns house will net you more rent as the further you go....meaning Snowspire Village will give the quickest rent/gold once the house is fully upgraded and you have placed a high valued trophy on display).

In my playthrough once I used this method in Snowspire Village (after I completely upgraded the house and placing a high valued trophy) I was gaining almost 10000 Gold in less than a minute.

Also note that if you do NOT care about killing villagers or if you are playing a evil character you can kill the towns people and buy their houses/shops as well, though these houses cannot be upgraded you can still gain rent from them. (so if you killed everyone in Oakvale and bought all the houses/shops while you are doing this gold method the rent will also build up for each property you own giving you more gold in 3 days....but note you will have to run to each property to pick up the gold bags making this process a bit longer.

Personally Knothole Glade & Snowspire are the best towns to do this trick in since there are houses right next door to your property that have beds next to the door. Oakvale does as well but once you get Knothole Glade you will gain a bit more gold than you do from the Oakvale Property.