When bending a branch to the ground, it will root faster if it's not broken. Just set a brick or rock on it to keep it in contact with the soil. Some leaves may need to be removed so the stem touches the ground well. This works for lots of plants, not just Syringa. If it won't bend that far, try bending it to contact the surface of a pot full of soil.
Pruning will cause suckering. Try not to trim much if you want a tree form. So few people let lilacs be the little tree they want to be. The key is to plant a single sucker and leave it alone. Unless you start hacking at it, it should rarely make suckers.
kqc - have you ever seen Nandina?
fiwit - how are the separated suckers doing? Lilacs and violets are on the top of my list of plants I miss from the north.
