I think officially you're only allowed 1 insert and they seem to be stricter these days. (And after about 100 pages, it's gets to be pretty bulky for a shirt pocket and you get tired of waiting while immigration officials spend 2 or 3 minutes trying to find visas and entry stamps.)

My old 96 page passport was filled (>500 stamps,visas,etc). So I got a new one earlier this year.

Easiest way (expense aside) to do this is at a U.S. Passport Agency, of which there are about 25 located in large cities. You're supposed to get an appointment, but it's easy to walk in. They'll issue same day if you have a copy of an international airline ticket departing within a few days. Turn around time is a couple of hours.

As soon as they handed me the new 52 page book, I took it back to the reception window and requested a 48 page insert, which I got in about 15 minutes. I figured it was easier than going back and making a separate trip in a couple of years.

P.S. - The main problem with a new passport is remembering the new number. Now I always have to pull out the new passport in order to fill out customs and immigration forms.