I used to put 1 oz of prune juice into one (and only one) of my 5.5 mo bottles a day. Now that she has been on solids for about 2 weeks now, I haven't had to do that for the past 4 or 5 days.
To echo PP's, prunes and pears are a good idea, and use bananas and cereals in limited quantities, if at all. In a few months, even that precaution might not be necessary.