Buying, cleaning, preparing and storing food for your iguana will probably be one of the most time consuming parts of caring for an iguana, as it probably should be. Just as much thought and effort should be placed in the preparation and storage of the food as choosing the diet itself.
Rinsing and washing - As always, it's important to rinse the vegetables and fruits you provide to make sure they're free of dirt and other debris. Just because you have a wild iguana, doesn't mean it has to eat "wild", dirty food.
Cutting, chopping, slicing & grating - It is believed that it is easier to digest smaller pieces of food than large chunks, so shredding and finely chopping most of the food is a very good practice. Larger pieces of food, especially hard vegetables, can impact (build or clog up in its digestive tract) or even puncture its intestinal tract. Most vegetables can be chopped and shredded by using a knife, cheese grater, or better yet, a food processor or Salad Shooter. Food processors have become quite affordable and can save you a lot of time. It's important to make sure the pieces of vegetables and fruits are small enough for the iguana to eat without any struggle. Feeding small, shredded, and finely chopped foods is a good idea, but the mixture of food should not be a mush. Leafy greens can be offered in larger pieces. Some of the harder vegetables such as squash, can be warmed in a microwave for easier cutting. One of the best ways of mixing food is to provide the shredded and finely chopped items in a mixture. Some people prepare food daily (which can be very time consuming) and others prepare larger batches of food in advance, storing it in ready-to-go portions in the refidgerator or freezer. Most generally, preparing about a week's supply of food is a wise choice.
Special care for leafy greens - After greens have been rinsed, it's a very good idea to use a salad spinner or towel to dry off any moisture. Even though many iguanas get a good supply of water from the food they eat, dry leafy greens last longer when stored as dry as possible. It's recommended that you store the leafy greens seperate from the other foods. Placing greens in a plastic bag and removing as much air and moisture as possible is a very good practice in keeping them fresh for as long as possible. Many people also store the leafy greens, wrapped in a paper towel, in a plastic bag. The paper towel will absorb the moisture, keeping the greens fresh.
Storing food - Whether the food provided is prepared daily or weekly, proper storage of the food will ensure that the iguana is getting fresh, quality food at all times. It's important to note that fresh food is good food, and when in doubt, do not feed any food that may have started to spoil. There are many different ways to properly store the food, ranging from sealed plastic containers (Tupperware, etc.) to plastic sealable bags (Ziploc bags, etc.). Plastic bags are recommended so that the air can be squeezed out, keeping them fresh. Generally, the food will remain fresh in the refrigerator for about a week.
Freezing food - Many people prefer to prepare a larger amount of food, freezing it and thawing it out when needed. If larger batches are prepared and frozen, generally, more than a one month supply is too much. Freezing breaks down the thiamin in the food so frozen food should also be supplemented with crushed thiamin (vitamin B1) tablets or brewer's yeast.
Iguana Care, Feeding & Socialization by Melissa Kaplan is an excellent guide to iguana care. The entire article, as well as her entire site, is an excellent source for all iguana owners.