Although there have been lots of talk about
training movements, not muscles, there are still times when we
do isolation exercises to strengthen certain isolated muscles.
These exercises are preferably done after all the compound
exercises to muscles in which you might feel that you want to
improve.
Problem is, how do we make these isolation exercises more
effective? I guess everyone knows the answer, squeeze hard at the
end of each rep, ie contract till you cannot contract anymore. Done
correctly, most people would find it hard to do more than 10~15
reps in a set for crunches. Yet how many of us actually do this
"squeeze"? 
Anyway, Merry Christmas 