Always under-promise!

You should always under-promise when discussing a commitment or project.  Being ambitious starts with wanting to achieve a new state.  How to get to that state and the path and time needed to get there will always be more complicated than you initially expect.  You move from being optimistic and venture into the land of the naive to expect it to be easier than you anticipate.  I have always found that you manage expectations better this way.   The negativity that you get from missing on delivering something in line with expectations is large.  It is also equal to the gain you get when you can over-deliver with the same outcome but when the initial expectation was lower than the final result.  

This is not a challenge to be lazy or ignore work; the effort put into the task should always be the same: 100%.  But more realistic planning and promises will lead to more of your hard work looking like a success rather than a near miss, and more of your colleagues and friends seeing you as capable rather than unreliable.  Of course you will find those people that are never satisfied and have unrealistic expectations no matter how much you try to show them what is actually possible.  Not sure how to help those people.  But definitely don't be brow beaten into committing to something that you can't complete; that is only going to end up bad for you.