If your body will allow it, do it now! Milk can be frozen for up to 6 months in a deep freezer and frozen in a dual fridge/freezer for 2 months. It can also be refridgerated for a week. Get the storage bags, it makes a world of difference. Try to pump in between nursing so you can build up your supply as well.
Now! Pump immediately after the baby eats. This tricks your body into thinking the baby is consuming more, and will help you produce more. Like the other mom said, store it in the freezer and make sure to date it. Also, if you happen to skip a feeding for whatever reason, pump. Once you go back to work, pump at every opportunity you have to help keep your supply up. Good luck!
I would do it now. I would also start using the bottle now too. Not all the time, but at least introduce it. Your baby might not accept the bottle from the sitter because he/she is used to feeding from you. So you'll have to get him/her used to doing both and the sooner the better.