Take the cue from those around you. If something comes up you'd like to elaborate on, take advantage of it. Pay attention too to the body language and non verbal communication of the other person to gauge whether they remain interested in the topic. Socialization and communication is so much more than merely what is said.

Ask open-ended questions and avoid closed-ended questions (ie ones that can be answered with yes or no). This way you are inclined to maintain a conversation.

Phrases you might use are:

What do you think about....
I have been considering...
There is an issue I'd like to bring to your attention...
When is a good time to discuss...
I would like clarification on...