I want to trim my lilac bush. What is the best time of year and conditions (leaves or no leaves)? ...

I hold two Hydrangea bushes, and they will...
Answers:    Lilacs flower on buds found on the tips of the branches. Pruning now, especially tip pruning will get rid of the flowers this spring. However, it is easier to see the mess inside the lilac now. You can prune at the groundwork only by removing the oldest stems at the ground horizontal (the wood is very rugged, your pruning equipment better be sharp) and pull it out. Also you can remove a even-handed amount of the suckers that have be coming up these last few years. Above adjectives, NO CUTTING on the tips of the remaining branches. This will save flowers for this spring. After the plant blooms, you hold about 90-100 days until that time new buds are formed on the tips near the flowing year's flowers. So it's OK to tip prune right AFTER flowering up until July 4th or so.
Tall/fast growing plants for privacy?
trim it when it is dormant, basically postponed fall or rash spring.