Do something that really excites you, that makes your finger tingle when you think about it.
For me it was building or designing something that goes into outer space and thus I bit my way through university to become an aerospace engineer. Without the excitement behind that idea I wouldn't have survived the curriculum.
And now I might designing something that will go to Mercury in a few years! YAY! ^^
(it's still open if we win the contract)
Long story short: Always look for a goal that excites you and then go for it.