In some cases there is not a lot you can do about it. If your weather is regularly hot and humid mildew will flourish. Lilacs are particularly prone to mildew.The only things I can suggest are try and make sure air can freely circulate around your plant (Do not plant up against a wall or plant shrubs too close together)When pruning try and maintain an open structure. Make sure it is getting enough sunlight. They will tolerate shade but are less prone to mildew if out in a sunny position. If your soil is on the moist side try to add something to make it more free draining and never water overhead with a sprinkler etc. Always water directly at the base and try and avoid splashing water onto the plant. Do not fertilize whilst suffering from mildew.