Include units at every step

There is probably no habit that a physics student can learn that has a higher return on investment than including units in calculations.  To see what I mean, consider this routine momentum calculation:

Can't remember the units for momentum?  Don't have to, they're already there on the page!  This is particularly useful if you are not using base SI units: you don't get confused or forget which units you're using along the way.

Better than that, this method gives us a powerful way to check our own calculations.  In the next example, I calculate the time required for an object to fall a distance:

What!?  Metres per second isn't a unit of time!  I've made a silly mistake re-arranging the equation; the kind of mistake it's easy for a student to make, and even pros do occasionally.  But pros tend to catch such mistakes because they have kept track of the units.  That means we can fix it:

The unit being right gives us some confidence the number is right.