I think you have two options or can use a combo of both. Without knowing more about your business or site, it's difficult to be very specific, but here goes...

1) Make sure the chat feature is clearly visible from every page, but not obstructing normal navigation.

2) Program live chat to pop up at key points in the buyer's journey. For example: If someone is lingering too long on a page, you could ask if they need help. If they are abandoning their cart, you can try to redirect them. You could even use live chat to suggest a complimentary item.

However, if you find that you need live chat in order to complete the majority of your sales, you may need to reassess your website to ensure it's doing the heavy lifting you need.