my son was a big nighttime feeder and at 22 months he was still feeding every 2 hours at night, but not at all during the day! i used to frequent a 'no cry sleep solution' forum on yahoo and someone there was using a gentle night-weaning method which worked brilliantly for us. we were co-sleeping, but you may be able to adapt it to work for you...

basically i continued to feed ds when he woke at night, but the catch was, i sat up in bed so he had to stand or kneel to reach the breast. after a couple of nights, during which he fed for a minute or less at a time he decided it was too much effort and just stopped waking for feeds. I then started to offer a sippy cup with milk before bed, and about a month later he happily dropped the last remaining bedtime breastfeed, so for us the weaning process was painless and easy.

can't really help you with the daytime feeds though! but at 20 months you can certainly explain things to her, like that your bbs are tired, or running out of milk, or whatever explanation you want to give - and that she's a big girl now and gets to drink from this special cup (buy a really fancy one) etc etc.

good luck!