Oh this is a tricky one, and a good question. I can have trouble with this depending on the type of people involved.

I like Jennifer's traffic island analogy. It is kind of watching for those pauses and interjecting if it feels right (not fretting if it doesn't). I do better with people who are gentler in their style, who will include those who are quieter with subtle prompts - it is often harder with more talkative people to get a word in edgeways.

Asking people polite and sociable questions about themselves during lulls is always a useful conversing strategy (ha! that sounds so formal but I have no better way to put it).