Once you are pregnant and while you are still working stock up on expensive things like toiletries and cleaning products. That way you won't have to spend as much on groceries once you are off work. I stocked up on cleaning stuff and toilet paper etc before I went on maternity leave with DS1 for a year and we didn't need to buy anything like that the whole time I was at home. Definitely made the grocery bill smaller!
I agree with stocking up on frozen meals and snacks even. Do some big cook ups and freeze things into portions. You can freeze muffins and things like that too for an easy snack. Very useful when you have a newborn bub too!
Buying second hand baby items is a big money saver. They don't need everything brand new. You can pick up some bargains on gumtree and local buy/sell Facebook pages. You can also make some money by selling things you no longer need on these pages too.
Buy nappies when on special, and baby wipes. Or if you want to save even more then perhaps look into using cloth nappies once you have a bub. Massive savings once you spend the initial outlay of buying up the nappies. A friend of mine makes her own baby wipes (take a look on Pinterest and I'm sure you will find the how to). You could also make your own cleaning products to save more money.
Shop around for your groceries, fruit and veg if you can. Woollies and Coles aren't always the cheapest option for fresh produce. See if you have farmers markets in your area that you could go to. Stick to a list when grocery shopping (meal planning helps) so that you aren't spending extra dollars on impulse buys. I find doing it online I don't have the chance to impulse buy as much.
I have been a SAHM pretty much since DS1 was born. I did go back to work for 3 months after my year off with him but I was already pregnant with DS2, so then went on leave again and haven't gone back. I'm very lucky that my husband earns enough that we can live comfortably without my income. We have a mortgage and car loan, the. Just regular bills. I have everything direct debited monthly or fortnightly. Some money goes directly into savings account when hubby gets paid. I put a set amount of money on big bills like water, electricity and rates every fortnight so when the bill comes I am in credit or only have a small amount left to pay. The only large bills we get are the vehicle registrations.