"Less or fewer".
A tricky one that. 'Fewer' refers to a smaller number, 'less' to a smaller amount.
Some will try to clarify it as whether it relates to a number (in which case it's fewer). But this falls apart with sentences like "I have less/fewer than ten pounds", where, even ignoring the fact that 'pounds' can also relate to weight, both words may be accurate.
If I pull a handful of coins from my pocket, I may be checking the value I hold, and could say "I have less than ten pounds."
Alternately:
If I pull a handful of coins from my pocket, I may be checking whether I have enough coins for the meter (or for the pinball machine), and could say "I have fewer than ten pounds."

As with many of these linguistic problems it often helps to find alternate wording, either as a replacement, or to clarify the meaning.
In the two examples above I could have said (respectively):
"I have not as much as ten pounds"
or:
"I have not as many as ten pounds"

So the way I remember is that if I could replace it with 'not as much' I say 'less'. If I could replace it with 'not as many' I say 'fewer'.

Cheers
Nigel