C.O.
There are several ways to make money - but none are "guaranteed"...
sell the things you have and don't use on ebay, craigslist or at a garage sale (this is instant and not month to month).
If you have creative talent - you can sell things on etsy.com - which is a subsidiary of ebay.
I would also update my resume and make sure my skill set is up and start applying for better paying jobs. They are out there. There is A LOT of competition so you need to make sure that you are SPOT ON and start applying...you are looking for AT LEAST a $7K increase in salary if you want and extra $300 a month.
The other thing I would do is start cutting back at home. Keep your thermostat set higher when no one is home and have it start cooling the house off 30 minutes prior to your return.
use Coupons. Start menu planning off the sales for week from your grocery store(s).
Freecycle - you can get a TON of baby things this way...and it's FREE!!! You just need to go get it...so okay- it's not "free" because you are paying for gas - but there is NO COST to you for the product or clothes.
Cut back on cell plans, cable/internet service...
Verify that your home is energy efficient...no leaks in windows (why heat or cool the outside?)
Stop buying books - borrow them from the library.
Stop buying movies - rent them from the Red Box.
Stop buying Starbucks coffee...(if you do)
Bring lunches to work - that is a savings of almost $10 a day depending on where you eat....
There is a TON you can do to save money...in the scheme of things $300 is not a lot...but it is...and there are SOOO many ways to cut back and save...(like I changed to the Verizon All-Share plan and saved $50 a month on my cell phone).