First -
know what you are prunning!. You should be able to ID that the plant is a hydrangae, or a viburnum etc If that does not work, at least know when they bloom.
If the bloom in the spring, then WAIT to prune until just after they are done blooming. The flower buds have formed last season and prunning now will remove the flowers for the spring.

If they bloom in the summer ( like butterfly bush) then prune in late winter or very early spring. These plants will bloom on new wood or new growth.

The big challenges are the pink or blue hydrangaes! - Dont cut now - wait until after they bloom in the summer.
Check out your local garden center or county extension office - there may be conducting a pruning class that can be more informative.

Here is a link that might be useful: More Information on Garden Design