When do you prune a lilac tree?

I usually start with cutting out all of the 'suckers' in the bush. These are the very straight branches growing straight up. Then I trim to the height that I want to bush at. Then I will thin out a bit, trying to take out some of the very oldest branches to allow the new growth to fill in. Another criteria to look at is anywhere two fairly large branches are crossing each other, particularly if they are touching each other. The main thing to remember is that lilac flowers on two to three year old wood so don't prune the tree too hard unless you are prepared to do without flowers for a couple of years!