1. Get the timing right. If your company’s just got terrible financial news, or you were warned about your slow production last week, or your boss just separated from her husband, now is not the time to make a special request.
2. Make sure you deserve it. Do you have a track record of accomplishments and increased value to justify your request? It's pretty easy for a manager to say no to a request from an employee who isn't wowing anyone; it's much harder to turn down a request from an employee who she’d be devastated to lose.
3. Build a business case for it. Ask yourself why your employer should find your proposal attractive. For instance, if you're proposing working from home one day a week, maybe you'll get more done because you'll be working during the time you'd otherwise be commuting and will end up putting in more hours than if you were working in the office.
4. Preemptively point out the downsides and offer solutions. Pointing out the downsides yourself -- rather than waiting for your manager to do it -- can be powerful, because it vastly increases your credibility. Suddenly, you're not trying to "sell" your boss on something, but instead are collaborating to figure out how to achieve something. Plus, if you don't foresee the downsides and offer solutions to them, you're leaving your manager to resolve those downsides -- which makes your request much less likely to be granted.
5. Know your own power. If you're a fantastic employee, you probably have more power than you think you do. If you’re great, your manager doesn't want to lose you and is probably willing to go out of her way to try to accommodate you, if she can.
6. Realize the answer might be "no" for reasons that have nothing to do with you. Sometimes your request is reasonable and your boss would like to say yes but can't, because she's stymied by bureaucracy above her, or has to deal with five more urgent issues first, or knows that if she’ll face a revolt from others if she grants your request. Be sensitive to the realities of your workplace and take a broader view than just how things look from your own desk.
7. If the answer is no, find out what it would take to change that. For instance, if you're turned down for a raise, ask what you'd need to accomplish in order to earn one.