Don't be random. As you listen to a conversation, various related facts will pop out of your memory as you integrate the information. If you feel that the discussion would be clarified by your facts, try to compose a sentence to introduce them. It may be sufficient to just get started on that, and finish the composition as you speak, in order to get in before the topic has moved on. Many times, there is no opening, and you have to start over with a new topic. If you interrupt, try to do it as someone is finishing a thought, and is just tidying up, or before they have gotten well into a paragraph you consider redundant.