For a moment he couldn't think of how to say it.

After all these years, sometimes it's difficult to think of what to say.

Before she could think of what to say, he went on.

The truth was that he couldn't think of what to say.

Now he had until Monday to think of what to say.

He paused, probably trying to think of what to say.

He looked at her but couldn't think of what to say.

He settled back and tried to think of what to say.

In their absence he couldn't think of what to say.

She looked at him, trying to think of what to say, and after a moment he smiled a little.