1) Farm plots can be a neverending source of plants for food and booze production; set your orders to 'farmers only harvest' if you want to maximise skill gains on your growers.
1a) And if you're only using the underground plant types, you're missing out on most of the variety of farming. Herbalism (Plant Gathering) is pretty good at producing food by itself, although it can be hazardous - fortunately, you can make above-ground farm plots to accept the seeds once you've processed the plants in a non-destructive manner (ie. not cooking).
(Note however that Quarry Bushes, grown from rock nuts, are quite absurd in terms of food production - each plant produces 5 leaves once processed into a bag at the Farmer's Workshop - but they can't be eaten raw, so you'll then need to cook them)
2) Poultry farming is a good way to produce large amounts of food for almost no maintenance - take 3 turkey hens at embark with one turkey gobbler (substitute other egg-producers as desired), build a few nest boxes. For crazy high returns, forbid the first batch of eggs from each whilst they're still in the nest box. (Also a convenient source of meat, bones etc)
3) Alpacas are very versatile creatures - not only can they be butchered, but like many domestic farm animals they can be milked - this milk can be cooked in with solid food as-is or made into cheese (yes, those cheese-maker migrants do have a non-military function)
4) Hunting provides a way to turn trees into food by the intermediate product of wooden bolts. Caution: dwarves do not have a sense of proportion, and a Dabbling Ambusher may decide that he wants alligator for dinner, usually with the opposite result.
4a) Your military may be able to 'hunt' too, but they're not quite so stealthy so make sure they can catch what they're chasing - it's embarrassing to watch a burly Hammerdwarf chasing a wild turkey around the map and collapse from exhaustion.
5) Fisherdwarves seem to turn up in droves, if you have a water source containing fish they will happily depopulate it for you. Remember to build a fishery to turn nasty raw fish into slightly more palatable raw fish.
6) Cookery can be used to combine ingredients from any or all of the above. If you are very cunning you can try cooking 1 large stack of solid ingredients with 3 barrels/pots of booze, which produces a large amount of food from very little raw material. (This can be more difficult than it looks though, especially if your kitchen is near a thoroughfare as your dwarves will drink the booze out of the kitchen before your cook can gather all the ingredients)
7) Trade can get you large amount of food for any second-hand clothing that you happen to have lying around, say in pools of goblin blood, or any other plentiful trade goods.