When you have a high enough pickpocket skill, steal some treausure machines. Blue ones weigh 14 stones and it's the best way to raise your house rank. Place them in your home until your house becomes 1st then sell it. It's a good way to complete thieves guild stealing quest. Blue ones go for like 15k if I remember right. If you have a really high pickpocket skill and enough strength, try stealing the red ones.
Steal crystals if you don't have a high pickpocket skill. They don't weigh much and are valuable. My castle is 1st in rank and I got that by having 6 treasure machines and around 10 crystals in my house.
You can clear some dungeons to increase your strength. You have to use the combat skills that train strength though. If you are using Two Handed, Axe/Martial Arts/Scythe/Long Sword and Tactics for combat, clearing dungeons is the best way to raise strength. You'll get loot and chests too. If you have a Sand Bag, tie up a monster and just beat the crap out of it. It won't die anyway.
Or you can always visit a fountain and savescum the crap out of it until you get a STR increase.
edit:typo