Here is one of the most valuable thing I've learned at Y Combinator. First day. Our first office hour with PG. He talked to us for a while and asked us to commit to ONE specific metric that is going to be our indicator for success and traction in the next 3 months.
After some discussion we agreed that in our case (swipegood back then) this would be % of growth of fully enrolled users.
We promised at least 10% growth week over week and sealed that promise with a handshake with PG (we delivered 20% during YC).
That changed everything for us! It made us hyper focused. It made us ask anyone and everyone we knew to sign up. And once we started running out of friends we started thinking intensely about what it would take to keep the growth rate (and our promise) going.
At first you're moving small numbers. You grow from 10 users to 11 (wohoooo)! But you have to realize that you are signing up people for a service that nobody knows yet. As the number grows it gets increasingly difficult to keep it up and you have to start developing marketing and distribution channels and build a product that people love and that build momentum. And that's exactly the point!
The other thing it does is it helps you prioritize things (one of the toughest things in a startup). See when you fully commit to just one simple goal it makes it clear which todos are more important and which ones are not. You might think "well some things are more long term oriented and we can't just be so short sighted and not do any of them" but in the initial stages of a startup you shouldn't care about the long term. In fact it's dangerous to care about the long term too much. Chances are you'll never get there. You need to optimize for creating enough evidence that you deserve to live another day and that the resources that are being invested in this new idea are not wasted. Once you grow and build momentum you can start worrying about the long term more. It's all about momentum and timing.
So here is my advice: pick one metric (yes - just one) and focus on it as your compass to success. It's going to make all the difference.