Ok you may loose some flowers.  But please don't get hung up on that.  When you cut a plant it will naturally become more bushy - sending out multiple shoots at the point of the cut.   Keep the plant compact with plenty of pruning in the early stages and forget about flowers.  In the long run - you will have a stronger healthier plant. 

When starting out the best time to prune is now!  And prune for shape.  Always cut the hedge or plant - back to the green.  When I say that I mean when cutting a hedge - make sure there is some green shoots left.  This will give the plant a place to bush out and regenerate.

If renovating a leggy shrub - cut back to just above a bud if possible.  Otherwise cut it back severely and let it come back from the ground.  In many many cases the plant will resurrect.

And you will be thankful you took brave drastic action.