Don't be the first one to answer.
I always make it a rule—and even tell the group ahead of time—that I'm going to try to be the last person to answer the question. I even let them know I'm okay with awkward silences if they don't have anything to say at first. This encourages group members to answer and not wait for me to give them the answer.
Ask open-ended questions.
It's hard to get any discussion when questions require only a yes/no, agree/disagree, or true/false answer. If you're stuck with those kinds of questions, a great follow-up question that always works is "Why? What makes you answer that way?"
Ask follow-up questions.
Get in the habit of always asking follow-up questions, especially when people try to give a really short answer. There are all sorts of great follow-up questions you can use, like "Does that come easy or hard to you? Why?" and "When have you seen this truth play out in your own life?" and "What do you think is the biggest barrier to living that out?"
Pick some fights.
If I'm having trouble getting discussion going, I'll sometimes tell group members that I'm going to play devil's advocate and push back on their answers. Or I'll ask a more provocative question. Instead of asking, "What did Jesus call his disciples to go do?" I might ask, "Why would Jesus trust these guys when they didn't earn it? Wouldn't it just be easier to do himself? Is it worth the risk?" Another standby I love to use is "How would you explain this to a non-Christian who doesn't buy it?"
5. Sticking Together Too Long
This could be the most common mistake of all—and for good reason, because it's probably the toughest. If building authentic relationships is the goal, how does multiplying a group fit in?
Sometimes a small group will stay together with all the same members for years, even decades, and it can feel like a great thing because the group possess relational depth. But in the end, it can actually be tragic.
When we stick together too long, we deny this authentic community experience to others. Nobody in the group gets the opportunity to develop into a leader. The group members become stuck in their comfort zones and forget the importance of welcoming and loving new people, losing any missional focus. Group members lose the opportunity to hear fresh perspectives on God's Word from new people.
Throughout Scripture, God consistently commands his people to multiply. Jesus himself left this command as his final great commission in Matthew 28 and Acts 1:8. It's a non-negotiable. Healthy things are meant to grow. As our small groups grow as we invite and include more and more people, we have the opportunity to multiply and create new groups so even more people can experience this kind of community and life change.
Multiplication doesn't need to kill the deep relationships and community that a group is experiencing because there are lots of different ways to multiply a group. A group can become a place where future small-group leaders are invited in, developed, and then sent off. Or groups can send off just one or two couples together that have some sort of affinity (maybe geographic or life-stage) to multiply a new group.
I have multiplied my small groups many times. And while people always fear it at first, they end up excited about it and cheering it on. How? I constantly remind them of the reason for it. From the beginning of the group, I cast vision for multiplication. When we send off group members to start a new group, we lay hands on them, pray for them, and commission them. After the new group has started, I like to find ways to get the multiplied groups to gather together again for "reunions."