For me, I really wanted to try and continue breastfeeding all the way to the one year mark, but I know that isn't always possible.
If your supply seems low the best way to increase it is to actually pump more, or get your little one to nurse more in order to stimulate production. Also make sure you are drinking PLENTY/TONS of water. When I noticed my supply had dropped, I also noticed that I wasn't consciously drinking near enough water. We use up so many calories as a breastfeeding mom, so make sure you are replacing those!
I think after a few months your baby has probably had a good chunk of the benefits of breastfeeding, but I know the added benefits continue the whole time you do breastfeed. If you can keep going, it is such a wonderful thing for you and the baby.
I had a heck of a time getting started with breastfeeding, but the E. Blackwell Center, right behind Riverside Hospital, was a tremendous help. Don't hesitate to seek advice from them. They have a free breastfeeding hotline you can call, and you can also see a lactation consultant for an hourly fee. I have used both and was very grateful for their help.
My motivations to keep going were.......I think formula smells funny and I didn't want to have to deal with that. Also formula is SOOOOOOOOO expensive. Breast milk is free!! When breastfeeding I have found that my baby weight came off so much faster.....you are burning all sorts of calories through milk production! Also, in the middle of the night when you can stumble into the baby's room, half asleep and just breastfeed rather than having to wake up and make a bottle is a huge plus. But I have heard that a little formula at night tends to stick with them a little longer than breastmilk.
There is nothing wrong with supplementing with formula, but those added benefits of continuing to breast feed are wonderful too!
Good Luck and hang in there!