Even from the very beginning of the game, Food is one Resource that weighs on your mind more than any other. It runs out quickly, your upkeep can easily run into the negative, and if you don't keep your Net Food Production in the black you will find yourself dropping further and further behind in building upgrades, Lost Arts, and Unit production.
As always, thinking about what expenses are coming up before they come up will go a long way towards keeping your queues, Craftsmen, and House of Scrolls busy.
Additionally, keep in mind that the object of the game isn't to keep the biggest possible collection of warriors sitting around at your Castle; warriors are meant to war.
Food is relatively easy to get hold of. The method that requires the least effort is building and upgrading Farms in your Castle. 6 Farms, close to the current maximum level, should be the center for all your early Food production.
Of note, the only ways to multiply your Food Production are with Paragon Status and the House of Unity. As a result, if you want to get the most out of your Farms, spending Sapphires on Timber and upgrading your House of Unity is the way to go.
Raiding sensibly will get you much more Food per day than your Farms ever could and knowing how to raid for Resources will be vital for mid-late game when you are needing a hundred or two thousand bites of Food to increase just a single level of a Lost Art. While I have covered the topic elsewhere, I'll just reiterate that spying on a Castle before raiding and targeting abandoned Castles of a level above 50 will help you get the best return on your Raids if you are specifically targeting Food.
Settlements are a volatile and potentially expensive source of Food, attempting to retain one during PvP tournaments and quests is probably not the best idea (with a caveat that I will come to later).
As always be aware that the rebuild cost of a Unit may outstrip the return on Resources.
Additional minor sources of Food are Amelia's Gifts, Spoils of War, Gifts from Facebook Friends, and Food Packs, but as these aren't particularly consistent/sizable or guaranteed I wouldn't make them a part of your planning.
Once you have access to the Spring of Life it should become one of your highest priority buildings. The time spent upgrading, and the size of the reduction in consumption (4.5%/level) makes it a better use of your Resources than upgrading your Farms to the next level does for all but the smallest armies. Do be aware that the Spring of Life requires a fair chunk of Resources to upgrade so if you are already struggling with Net Food Production you will need to fix that before upgrading.
For those who have access to a Beacon and are of a sufficient rank in their League, you can convert some of your Units to Darkened versions. These Darkened Units consume less Food than their undarkened counterparts.
These Units consume half the Food of their normal counterparts. Eldritch Units can be gained through Tournament Rewards and Saga Quests. Losing upkeep hungry normal Units in order to gain Eldritch Units is a good trade.
By spending Skull Runes on Food Consumption you can reduce your warriors' food upkeep by up to 10% on top of what your Spring of Life gives you. Additionally, by spending Runes on the cost of Darkening you reduce how much Darkening you need to Darken your Units. This means you can apply even more, and reduce your upkeep even further.
It seems to escape so many players that a dead Unit eats no food. While I've touched upon this topic earlier here and elsewhere I'm going to spell it out for people, especially for those who currently have negative Net Food Production and are struggling to upgrade Buildings and Lost Arts.
Throw them at Battlegrounds or defended Settlements (use them to capture and then hold a Farm Settlement for the double whammy of making and saving Food) or even wisely chosen Beacons. Certainly do so during Tournaments or League Quests in order to maximize your return, but send your warriors off to their deaths, and then you will see your Net Food Production increase, and then you can go back to upgrading your Buildings and Lost Arts, and yes, building more Units to send into battle.
Managing your Net Food Production is easy! You can do it just by increasing your intake,
reducing your upkeep and planning ahead. The smarter you manage this the sooner you won't have to worry about it because all you'll have left to use it on is your next army.