Just turned 40, Singh Cassidy has landed a new gig: as the CEO of Polyvore, a three-year-old Internet outfit with designs on conquering the fashion and lifestyle space. As I mentioned in Tuesday’s Postcard about her news, Singh Cassidy told me that she has, for quite a while, kept a list of career guidelines for herself. They’re basically about understanding yourself, assessing your passions, and knowing when and where to leap. She offered to share them on Postcards. I gladly accepted. So here is some great advice from a Silicon Valley star who I advise you keep your eye on.–Patricia Sellers

1. Decide what you’re trying to prove–and to whom.
You need to park what other people expect you to do. Take it out of the equation and figure out what you’re trying to prove to yourself. Then parse it even further. If  the journey itself gets you excited, you’re on the right track. But if you’re trying to prove something to yourself–and if the very thought of how you go about it is draining–pause and reexamine it.

2. Believe in the “gut/instinct, then data, then back to your gut/instinct” cycle.
When you see something that you like/love, you don’t need to jump right away. But get as much data as you can. Then realize that the data is helpful but ultimately imperfect/incomplete and will only tell you so much. Use it for what it’s for: to “reinforce” or “validate” your gut instinct, or not. Then choose to take or not take the last leap of faith on an informed basis. At that last step, it will always be a leap.

3. Recognize that you can’t manufacture consumer addiction, or predict it, or overestimate its value.
A beloved user community that believes deeply in your product or service is your most precious asset. Treat it as such. Many companies have safe profitable business models, but one of the reasons that the consumer space online is so volatile–and so coveted–is because you just never know what people will love and use your product for…until you put it out there.

4. Look for lean teams that started on a shoestring.
Lean teams get traction very fast and at very low-cost. That DNA is the basis of successful, flexible, efficiently run organizations. I’m officially employee No. 13 at Polyvore, but you’d never guess from the big share of mind it’s getting among online users that the company is so small.

5. Look for the passionate product-driven founder or visionary with whom you can partner.
If you’re a business person in Silicon Valley, this is even more essential. Most of the great consumer companies here are driven by a passionate product visionary– most often a founder. This is someone whose consumer instinct is incredible. They also happen to be the guys–or women–who are most capable of inspiring and recruiting great engineers who want to work on hard problems. Who’s that partner for you? Welcome/embrace him or her. They’re key to your success as a CEO.

6. Similarly, embrace all the founders.
The founders are the people most obsessed and willing to do anything to make their vision and the company successful. If they’re all gone, perhaps you should be worried about a company run by the “professionals.” After I left Google, people often asked me if I still believed in the company and would ever think about selling my stock. My reply was that the event that would most worry me would be if Larry Page or Sergey Brin left. This is the same way investors would feel if Steve Jobs left Apple .

7. Figure out how to make the “unknowns” known before you say yes.
Most people spend a few hours, or maybe a day or two or three at most, with the folks they’re going to dedicate the next X years of their life to. (You don’t pick a spouse this way.) The biggest difference between success and failure may be your culture fit/synergies with the existing leadership team–rather than your competency or theirs. If this is the case, how do you get a look inside the company before you’re inside? Can you join a management meeting or a board session, meet more staff, or spend time with the CFO?

8. Look at what it the job is today and make sure you love it, vs. what it needs to be to get you excited.
It’s nice to  love a company for what you think you’re going to make it. And obviously, going to something early, you have the ability to shape it. But if it’s already got customers and employees and a brand, you’ve got to believe in the core of where you’re starting from. If you’re thinking about taking the company’s current assets and reworking a significant part of the consumer front-end experience, you actually may be better off starting from scratch to achieve to your vision. Most often, you want to play off and accelerate the natural flow/culture of the company and its customers.

9. Go in with a vision of your 100-day plan and see if it resonates.
Before you accept the job, you should know what it might look like in the first 90 days. Have a thesis or basic game plan–some sense of what you think needs to be accomplished. Vet it/check it with the founders and board. Also, ask yourself what your day-to-day job will be and how it will feel to do those tasks. How much fun is it? Is it energy-giving or energy-draining? Be honest with yourself.

10. Even when you start a new job with great ambitions of work/life balance, realize that balance will fly out the window again.
I don’t believe in work/life balance if it’s measured over days or weeks. I took the past year, had a baby, got perspective on the different trends going on in the Valley, took a couple great vacations with my family and enjoyed not having any employees. But at the end of the day, doing exceptionally well is usually the result of obsessively focusing on one thing and getting passionate and inspired by it. Use the off-cycle to rest up, recharge, and put in place support for the coming phase. I lined up more household support, got my core fitness level back, and actually gave up a couple great professional opportunities to take stuff off my plate. I’m entering a cycle again where my balance will be off kilter for a while. I can’t wait.