Jennifer, for the most part pruning is best done in the fall after the leaves are gone on deciduous trees and shrubs. Winter is fine too, and as long into early spring as the plants are dormant. Once you start seeing new growth, I wouldn't prune any more unless you need to do some shaping in summer if branches are getting in your way.
Clematis have 'special needs' though. You should wander over to the Clematis forum (click on this link)
http://garden.org/forums/view/... and post your pruning question there because there are I think three different methods for different kinds of clematis.
To avoid the very common, but SO wrong practice of "crepe murder" on your Crepe Myrtles I think
@dave is your guy for a link on how to shape and thin those lovely trees properly.
Forsythia is a vigorous shrub and as I recall the best method to keep it from getting too big and sprawling is to cut out two or three of the biggest (oldest) trunks right at ground level each winter while the plant is dormant. At our house in Utah, I had a Forsythia that had been viciously pruned into a ball shape for years, and after I began thinning it properly, it responded nicely to make a natural cascading shape again. Be aware that the spring bloom on Forsythia happens on last summer's new growth so that's why you don't want to cut too much off at once and don't cut from the ends of the branches. If you do, you won't have much bloom.
