$400 a month is a decent amount for one person's groceries. If I were making double minimum wage I'd probably try to cut that down.
You basically are saying you want to make more money but don't want to inconvenience yourself at all to do it, and it really doesn't work like that.
It's always best to start some sort of side business if you want extra money. It's way easier to clear decent amounts working for yourself than it is flipping burgers or whatever.
Think about something like mowing lawns - your hourly rate on something like that is going to be $20-$30 an hour before expenses. Obviously getting customers is always the hard part on service businesses, but there you go. The problem being, you'd actually have to work outside of your normal job, which you seem unwilling to do.
Anyway, I recently found these two articles interesting, and many of the ideas on the lists can be done part time: