Here are the settings that work for me and I am getting a full day with my standard battery.
For Battery Life: WIFI always at home. CDMA unless I need the fast data download or am in a solid 4G area. Set your phone on "Normal" not "Performance" this phone is fast anyway. Also I like to prep my battery by fully charging it as soon as I get the phone then fully discharging it for a couple of cycles, then random charging a few times then a couple more complete discharges. Now some people will say this is not necessary but it works for me and as always YMMV.
For data use conservation: Use WIFI whenever you can, get to know heavy data usages like Facebook and picture files and manage them carefully. Then manage your use accordingly, for example, don't just text back or answer FB immediately, don't have your phone set on auto sync - do it manually. Also straight calling does not use data so call whenever possible. Think about what you need before just surfing the Internet.
300 MB a month equals roughly 75 MB per week or 10 MB per day. If you think through all your uses and budget how you use the phone for that 10 MB per day you will be fine and by reviewing the use per week you won't go over monthly.
You are gonna love this phone, good luck!