I never really got this concept. Aren't you spending money on unnecessary things like the real estate license, just to reduce your taxes by 28% of what you spent? So you would be better off not spending that money in the first place.
Plus the IRS is not stupid. If you continually run a business at a loss they will come and audit you to see if you are actually running a legitimate business. Then they will also find any deductions you made that are personal or not related to your business at all, and disallow them.
So my follow up to this is here:
1) It only makes sense if you start a business that actually has the potential to earn you money AND the depreciation/write-offs you take are for items that you were likely going to purchase anyway (more on this later).
2) The "rule" is 3 out of 5 which has been debated here previously, in terms of profitable vs. unprofitable years. Face it, most businesses are unprofitable for the first few years, the IRS knows this. I put rule in quotations because it's not like year 4 of unprofitability, the IRS shows up to your door and disallows all of your deductions. My example was Virgin America which has lost money in consecutive quarters from 2007-2013. Which leads me to...
3) You do not need to establish an LLC or S-Corp in order to capture some of that deduction cheese I'll talk about. The only reason you would do this is to shield personal assets from liability claims against your business. Specific to California is the $800 fee that's really a sunk cost and keeps people from incorporating for the hell of it.
4) In terms of deductions, if you want to start a tractor business as your side-business but you live in Los Angeles and have never stepped foot on a farm...stop. If you're a geek who likes tinkering and buys the fanciest of fancy computers/gadgets.... starting a consulting/tech support/web design company might be a better plan, especially if you were due to upgrade all of your regular equipment anyway. Budding photographer? Start a photography business. Crazy about calligraphy? Go for it.
Items purchased for a business are deductible, though the requirement is that they are primarily used for the business. This rule is more or less unenforceable (is the IRS really going to log on to your business laptop and ask to see your browsing history?) My accountant advises me to be aggressive in what I consider a deduction because a) audits aren't really that bad and b) over time, unless you're full on committing intentional tax fraud, the value of those deductions will exceed the costs and penalties associated with an audit (most audits are just requests for information/documentation anyway....via US mail).
She says as long as I don't try to pass off a luxury boat as a business expense, I'll be fine (actual client did that for their RE business... deduction was disallowed at audit).
5) There's really no way for the IRS to differentiate someone who starts a business with the intent of failing vs. someone who just really sucks at their business. They can't read your mind. Just don't be egregious about it (boat example).
6) If you follow these general guidelines, a side-business is a great way to a) have additional income potential and b) a source of legal tax deductions in its formative years. Just be sure to CYA with appropriate licenses, local permits, and reporting of sales tax to your respective tax board. You can even massage your business to be profitable in enough years just to keep the deductions rolling (see my comment in #5 above).