$300 for groceries is not unreasonable, but I'd bet you don't even look at the weekly ads to see what's on sale and that you buy national brands over store brands more often than not.
The wife and I started really paying attention to our food budget some time ago and it has really paid off.
Some advice:
1. Shop the sales, but don't buy something you won't eat just because it's a deal.
2. Buy store brands when you can - for some items there is no substitute (Pop Tarts, for a bad example) but for most items (pasta, canned goods, frozen veggies) there's very little or no difference.
3. Stock up. If you can buy a whole boneless pork loin for $1.50/lb instead of buying two chops for $3.29/lb - buy the whole loin. The meat counter will slice it for you and you can wrap & freeze what you don't cook right away. In addition to saving money in the long term, you also have a stock of food if you need it. When your favorite cereal is on sale, buy three or four boxes and store them.
4. Don't be afraid of coupons. If you don't want to pay $2.50/week for a Sunday paper, check out the wonderful world of free online coupons. If your store uses a "club card", check out the store's website. Some stores have e-coupons that you load right on your card.
Free e-coupons:
www.cellfire.comwww.pgesaver.comFree printable coupons:
www.redplum.comwww.coupons.comwww.smartsource.comwww.couponmom.comThere are lots of manufacturers that offer printable coupons on their websites. If you have a favorite product or three, check the companies website - you might be able to save some money.
If you don't get the local paper and the accompanying flyer for your local store, check the store itself. Most stores have a pile of flyers near the front door and there are often coupons in these flyers. Don't be afraid to whip out your EDC knife and cut coupons right in the store.
A caution about coupons: don't buy foods you wouldn't normally buy. Find coupons for the stuff you already eat and save some money. Don't be fooled into buying something you don't want or need just because it's forty cents off.
OTOH, don't downplay the value of $0.50 coupons. These things add up. Fifty cents here, thirty cents there - the next thing you know you're saving $5 on your grocery bill. $5/week is $250/year - nearly a whole month of "free" groceries.
ETA - if you can use a coupon on an item that's already on sale, that's the best. Sometimes you can get things for free after coupon.