Water intake
While it should be a no-brainer, water intake is another place where trainees make basic mistakes (I am guilty of this myself). The effects of dehydration range from minimal (at 2% dehydration, strength and performance decrease) to painful (can anybody say kidney stones) to worse (at 10% dehydration, death can occur).
While there are many generalized water intake equations (such as 8 glasses per day), these may not be correct for everyone. To poach a guideline from a friend of mine, a good rule of thumb is 5 clear urinations per day, and 2 of those should come after your workout. This gives trainees a way of individualizing water intake. Obviously someone who lives in a hot, humid environment (or trains in a non-air conditioned gym) will need more water than someone who lives in moderate temperatures and trains in a posh gym.
Water intake should ideally come from water and water alone. However, other sources such as milk, fruit juice, or fruit and vegetables can count towards total water intake as well. Anything with caffeine in it doesn't count because the caffeine will act as a diuretic. As well, alcohol tends to further dehydrate you so beer after a workout isn't a good way to increase your fluid intake. Oh yeah, thirst is a poor indicator of hydration state. By the time you're thirsty, you're already a bit dehydrated.