This year I was lucky enough to go to Startup School hosted by Y Combinator! Apart from getting to see many of my good friends while staying at Stanford, I met a ton of interesting people, learned some valuable lessons, and finally experienced the startup culture of the Bay Area. Here are some of the most memorable lessons from the day:

Phil Libin (Evernote):

build something you love

Choosing your cofounders is incredibly important, as you will be a huge part of each others lives for a long time.
Success as a company only makes things harder - the most fun and least stressful time is ironically when Evernote could have gone out of business at any minute. It is not fun day-to-day, but it is fun looking back month-to-month.
5 years ago, it would have been stupid advice to say 'build something for yourself'; today, it would be stupid not to. If you like something, chances are 10 million people somewhere else are as weird as you and will too. Building something you want gives you the unique advantage of knowing when it is truly great or not. Make something sufficiently epic to be your life's work.

Dan Siroker (Optimizely):

create your own algorithm for success

Ultimately, building a company is about the feedback loop - you make something, see what people like/don't like, and adjust.
A good way to test potential board members is to have a mock board meeting; you might be surprised with the results. Having people who ask the right questions is more valuable than people who give the best answers.
Most successful entrepreneurs don't follow some fixed algorithm of previous success; they make one to their own advantage. Best example of this is Elon Musk with Tesla by bypassing the traditional success algorithm of auto makers and cutting out the middleman.

Ron Conway (SV Angel):

look for a rifle focus on product and decisiveness

A successful entrepreneur is someone with a rifle focus on the product and is very decisive. Hire fast and fire fast.
Always follow the metrics - if users like your product, it is a good product.
Get one term sheet asap and use it as leverage to speed up the process with other investors.

Chris Dixon (Andreessen Horowitz, Hunch):

know a secret

Want to be in the sweet spot of a good idea that seems like a bad idea. Challenge a social norm. Powerful people will dismiss it as a toy or hobby.
Successful startups usually come from unbundling functions done by others (newspapers, schools), hobbies, and challenging social norms
The best ideas come from direct experience and domain expertise.

Diane Greene (VMWare):

have a big vision

If you think of a good reason for why someone should help you, they usually will help you (like Dell did with VMWare).
Bringing something to market that a few people love is much better than a product that many people like.
Having deadlines helps with everything.

Balaji Srinivasan (Counsyl, Stanford Lecturer):

voice vs. exit

To make change, you decide to either use your voice to create it from within or exit to create it externally.
Exit is the reason voice has strength (like a BATNA in negotiations)
Exit normally amplifies voice because it is radical.

Chase Adam (Watsi):

find something to work on that you care about more than yourself

Do well and do good.
Making the world smaller = making the world better.
Think about the long-term effects of actions - sometimes things that help people on day 1 hurt people on day 365. An example is humanitarian efforts giving rice in Haiti put all the local rice farmers out of business.

Jack Dorsey (Twitter/Square):

actively make yourself a better person

Keep a list for everything you are doing and notes for people - it is a great memory device.
Have a list of do's and don'ts that you look at multiple times a day. Create habits to make yourself better.
Act professional to be professional and think about the small details - tuck in your shirt, stand up straight, be present.

Mark Zuckerberg (Facebook):

don't avoid mistakes because you can't - just keep the determination to push through

There is no secret sauce of why he was able to make Facebook - just a ton of determination. So much so they would have lockdowns when they felt someone else was doing something better.
Vision from the beginning was connecting everyone in the world.
When hiring someone, ask the question - would I want to work for them?

Nathan Blecharczyk (Airbnb):

making a startup is like training to go to the Olympics

You are going to fail more times than you succeed - perseverence and confidence are key. Be a 'cockroach'.
You can always pivot an idea; you can't really pivot your partners. Take time choosing who you work with.
Give it 100% before you quit.