Choosing the right bouquet holder is important. Fresh flowers pull a lot of water from foam. I generally use a Grande holder unless it's a very small bouquet.
Treat the water with a flower food. This is important. You can't use too much - it will not hurt the flowers. But it is a waste of product, so follow the directions on the bottle carefully.
Allow the bouquet holder to "float soak". Do not push the dry foam underwater. This can cause dry air pockets that can mean death to a flower head or two. It only takes a short time for the foam to fully saturate.