Do fewer things; do them better; know why you're doing them
calnewport.com
calnewport.com
-Robert A. Heinlein
I would suggest the exact opposite: do many things, fail better, be open to new possibilities.
The options become less less as you get older, obviously. In fact I think the best description of the change in career outlook as you get older is a switch from breadth-first search of new experiences to depth-first search.
I thought this blog had the answer. But then I relaized that this will not work in every major / profession. My girlfriend is doing lots of things and somehow suffers under the heavy workload. But I can't give her this kind of advice because (at least here in Germany) it does not apply to someone who studies philosophy and politics and wants to get into the best positions as development worker. Why? because nobody cares about your marks. You basically get a degree for the sake of the degree itself and for living a students life while doing as much extracurricular activities as possible. Internships, important roles in local divisions of NGOs and so on are almost everything that counts for getting the job you want.
Personally, I still think "Do fewer things; do them better; know why you're doing them" worked incredibly well for me and may help many other students, I is probably not for everyone and certainly not for every subject.
This core philosophy is very close to the spoken truth and highlights my biggest gripe with cargo cults:
Blind faith and regurgitation of process/teachings without a deep understanding of what and why it works.