Arrangements that are based in wool with a tray or pot may need topping up every couple of days - lift the flowers or foliage closest to the wool to see if it's still holding water. You can top up the tray and the fibres will soak it right up, just don't make it sopping wet.
Arrangements in moss or with a straw base, like wreaths, can be kept looking their best by gently misting with water. Take care to avoid spraying anything that's been dried, like orange slices and peels or pine cones.