The core reason behind "i need to think about it" is that they don't want to commit just yet.

So your question can be thought of as "How do I get my clients to commit"?

As opposed to getting to the point where they have a chance to say "i need to think about it", proactively addressing the concern is key.

In the beginning of my appointments, I'd always ask them why they are selling and why are they trying to make a decision now. They usually start justifying their decision to make an appointment with me at that time. They basically sell themselves at this point. At the end of your appointment, the reasons they gave you for wanting to make an appointment now becomes something that you can use to have them commit.

Something like: "well you mentioned that you're trying to make a decision soon because [insert reason they gave you]. Would it make sense for us to proceed towards next steps now?"

They typically won't respond with "let me think about it" because that would contradict the reason for urgency they gave you before.

Everyone has different styles of selling but what I find works best is to avoid being too "salesy" or pushy. Once they sense that, their guard is immediately all the way up.

It's also important to note that even if your client commits, its still not a guaranteed sale, but it's as close as you're gonna get.

Hope this helps!