I just can't get the food and water in the green. I got 34 food each time, but it just depletes as fast as I can refill it. 6 people working in both of them.

Both food and water rooms (3 each, merged) are level 2. I have 25 people.

It's tricky isn't it.

I try to make sure that I have upgraded each production room.

That the people with the best SPECIAL are working in each room.

That I equip some outfits that boost the skills (if possible)

That I curb unnecessary room building until I need it.

At the moment I'm training up 2 dwellers on their preferred SPECIAL skill to further enhance resource production.

Takes time and patience, I have 43 dwellers and I'm getting there, steadily.

I agree slow and steady. Here is http://i.imgur.com/JlGvyzr.jpg and it's pretty balanced for my 33 people. Recently on a http://i.imgur.com/RDV39Cw.jpg I sent some people out into the wasteland. While they were gone naturally less recources were consumed. I was able to see how many Dwellers my vault could handle and allow me to get my Food and Water up and keep it well above the green line. Then I upgraded rooms and let them all back into the Vault.