Getting past the food aversion factor is the main thing. There are a lot of edible things out there that most people would puke at the thought of eating. For example the fresh eyeballs from animals are a good source of saline (salt) which your body needs.
As for insects you can forget spiders for the most part, at least all the survival instructors I ever had said you should. Same goes for mushrooms. They don't provide much nutritional value and are a real risk is you don't know what you're picking.