Start writing novels on your computer. Its best to write novels as they take the least amount of time. When you write, your skill will automatically advance and if your skill is high enough, you will be able to write more novel types/genres. The most money can be made from "Romance Novels". You will get royalties for six weeks once you finish a book. With the "Writing" skill maxed out, you can make approximately 40,000 Simoleons every week, which is more than the 34,800 Simoleons you can get by purchasing all neighborhood businesses, and takes less time to achieve. In three game weeks you should have a very high speed of writing, 9 or 10 best selling novels and 100,000 Simoleons.
No matter how many times you use an object, if you sell it on the same day that you obtained it you can get all your money back. If your Sim is poor and needs body points or something else, buy a body machine and keep selling it back to get a refund of all your money.
EASY COOKING, FISHING AND GARDENING SKILLS:
This trick requires a TV with a Cooking, Fishing, or Gardening channel and a video game system. Have a Sim sit down and watch television. Switch the TV over to one of the previously mentioned channels so that the Sim can start earning his or her skill points. Then, have a second Sim go over and start playing video games. Notice that although the TV is displaying video games, the first Sim still has the skill meter above their head. Allow this Sim to sit there for as long as they please until they decide to do another activity of their own. After that, the skill meter should stay above their head until you start another Cooking, Fishing, or Gardening skill activity. As an added bonus, you can train two skill points at once. For example, work out and both the Athletic and Cooking skills will continue to rise. If you keep repeating the steps until each skill channel has been viewed, the Sim will keep learning the three skills as he or she goes about their daily business, and four when doing skill activities.