Here are a few hints from my 6 month experience of
doing exercise with 4 year old daughter:

1) What I found helpful is to do exercise together with
daughter, than it becomes an "family activity" rather
then an unpleasant chore.

2) Sometimes we do it with music, as it is more fun, but
music should not be loud so you can still hear the "sniffs".
Loud short sniff is the key to this exercise and should
be enforced.

3) We found that advertisement breaks in TV programs is
a good time to do exercise as you have all the family
available. Time is anyway wasted. So we turn off the
sound and start doing exercise. Typically we
can get trough 2 exercises / break.

4) I found it is useful to count every 8 sniffs with
something audible. It gives the kid an idea that exercise is not
infinite and we are making progress. While you can not use your mouth for
counting all the time (as you have to do sniffs) I make
a loud snap with fingers every 8 sniffs, than after 8x4 (that
is a "set") a loud clap and after 3 sets a 2-claps which means
particular exercise is finished.
Sometimes we do only 2 sets of each exercise but in severe case 3
is better.

5) To prevent daughter from getting bored we sometimes do some
different "bonus" action before each exercise, like cartwheels,
or a "new" exercise made up by daughter (also with sniffs, but
with some funny new movements).

6) Clearly explain to the kid that this exercise will help
him to get rid of that cough and attacks. Once it starts helping,
build on it and remind that it is important to keep doing it
to prevent coming back of the trouble.

7) Remind of other advantages of doing exercise, like getting
stronger, catching cold less then other kids etc.

Regards,
Yevgen