Oh, and always check your units – I've lost track of how many times I've been off by six or nine orders of magnitude because I started off in kilometers instead of meters and then squared or cubed that value... and if you have multiple interdependent questions, that's a good way to lose a whole lot of points, very fast.
You're right on the money with that. *anecdote time*
On a practice test one of my teammates made me, it asked me for the mass of a human (70kg, obviously memorized) in yoctograms. However, silly old me read that as "yottagrams" and instead of calculating 7E28, I calculated 7E-20. Unfortunately, the next twenty questions were all revolved around this number (ex. How many atoms of carbon would it take to add up to that mass?
or How many times more mass is that than that a mole of cats?
). My partner and I had a heart attack when we got a 0/100, and we considered dropping the event until our teammate noticed our error (It just so happened that we were off by about 48 orders of magnitude for every question, which seemed really fishy). Oh my god, imagine that happening at a competition...
Moral of the story: READ VERY CAREFULLY. And as Pheeny said, watch your units like a hawk.