HI there
we have a family of 4 (all eating) and our groceries in total are $150 a week.

I have a lot of staples in the cupboard ie (flour, brown sugar, cornflour, condensed milk, etc - and will bake from scratch for sweet things).

I will also shop around for specials - buy meat in bulk - i find the supermarket cheaper than the butcher although i wouldnt say it is nicer. I will always look for marked down items that are very close to their use by date and freeze them.

i will buy bulk as you said - 5kg potatoes, 3L milks, 20Kg wasing powder etc.

all the staples and things which really dont matter to use i will buy coles or woolies brand or homebrand (ie dry pasta, condiments, breads, spreads, sauces - or i will make my own).

Nappies are a killer! esp going from cloth to 2 in sposies recently. i have noticed this alot.
I will buy snugglers bulk on special - usually $18 a pack of (72/64 depending on size) and they are fab - i dont even think of huggies or other brands.

Snacks for the kids i will buy in bulk ie sultanas (homebrand) in 2 kg bags and bought good storage containers so they last.

Softdrink is probably the one thing we do buy in the junk food area (otherwise we avoid it unless it is atreat - birthday/party etc). and always and only get on special.

Is your $300 a week? as this is heaps to me! this would be a fortnightly spend for 4 for us.
we always have cereals, smawiches and meat and 3 veg for dinner

(oh we eat alot of rice as sides etc instead of chips or the more expensive options).

this isreally disjointed sorry but if i think of more i will let you know.

Oh if i bought lots of veg and it is getting a little old - i will steam it and freeze it in an 4l icecream container - then just reheat later - helps prolong its life rather than throw it out.