When you want to express more about a particular topic or provide additional information, "say further" is a commonly used phrase. However, there are a number of synonyms you can use to add color and interest to your dialogue or writing. You can sub in phrases like "expand upon," "elaborate," "additionally," "in addition," "moreover," "besides," "furthermore," "also," "not to mention," "what's more," "on top of that," and "in furtherance of." These phrases add variety to your vocabulary and allow you to convey your thoughts and ideas more clearly and effectively.