Here's a short video on how to prune petunias. I
hope it's clear enough for you. If you don't have broadband, let me
point out there are two kinds of pruning. The first is really
deadheading or removing the spent or dead flowers.
The second kind of pruning is really pruning and what we're going to do
is look behind the first flower and find the growing tip of the plant.
Pinch this off and the plant will bush out and produce new growth that
will make the plant look bushier and will encourage flowering.
So the trick here is to do this *before* the plants get long and ugly -
do it when they are looking their best. You see it takes a few weeks
for a plant to respond with new growth and flowers and if you wait
until the petunia is looking a little sad, then you've waited too long.
Having said that - better late than never. :-)