With non-Christians I can let opportunities to steer the conversation in a spiritual direction pass by. When in fellowship with Christians I can lapse into talking about common things, rather than the things of the Lord.
A good place to get ideas for both types of conversations is the Colson Center's "Next Steps" blog, http://www.colsoncenter.org/nextsteps. From the blog:
"This is the place to come to take action. Would you like to discover ways to begin practicing your Christian discipleship in a much more effective and focused manner? Each week, T.M. Moore, the dean of the Centurions, will share suggestions on how to put your Christian worldview to use in your world. We invite you to share some of your experiences here, so that others can also learn also join the conversation. "