Here's a list of the various things that, when combined appropriately, can lead to impressive harvest yields.
It seems one of the first assumptions by new gardeners is that loads of light automatically equals loads of buds. Unfortunately, it's just not that simple. Yield is equally contingenton a number of factors; light, temperature, humidity, water, nutrients,CO2/ventilation, genetics, etc. Think of it as an engine, with each factor of cultivation representing a single piston, sure the engine will run if some of the cylinders are misfiring or not firing at all, but to yield the most power from that engine, all cylinders must be firing in sync and at maximum capacity.
Temperature. Most cannabis plants will slow or cease growth when temp's get above 85F, or below 65F. Optimal lights-on temp for most strains is about 72-78F, with 5-10 degrees cooler during the darkperiod being a good rule of thumb.
Humidity. Cannabis does best around 455 H (relative humidity). During veg and late flower, however letting it drop lower during the final two weeks of flower is advised, as it will help prevent mould problems.
Water/moisture. Cannabis generally doesn't like "wet feet", or a soggy environment, so it's very important to have a fast draining soil/soil-less mix (or well aerated solution in a hydro garden). Wet or damp conditions can also lead to mould problems during flowering.
Nutrients. Cannabis will require a variety of nutrients at varying NPK ratios during its existence. NPK stand for; nitrogen (N), phosphorus (P), and potassium (K) - the three major nutrients used by plants. Simply put, your plants will need a fertilizer with more N than P and K during vegetative growth and fertilizer with more P than N and K during flowering. Using any well-known quality fertilizer applied per instruction @ ½ strength is a good place to start. Organic, chemical,or somewhere in-between is another choice to be made and is a totally personal one. There is a plethora of fertilizers on the market, but the best fertilizer is the one that's used properly.
CO2/ventilation. Plants require CO2. There is sufficient CO2 in our atmosphere to support massive bud growth, but when growing inside you must either have adequate ventilation (the volume of theroom exhausted at least once/5 minutes) to ensure that there is aconstant supply of fresh, CO2 enriched air or one must have supplemental CO2, which requires higher temp's and more nutrients to be utilized effectively.
Light. Typically, the more the merrier, but more light will create stronger water, nutrient, and CO2 demands on the plants. You must also have the proper spectrum of lighting as well as a means of efficiently reflecting as much of the light as possible into the garden's canopy. The norm is to use more bluish light (Metal Halide, cool-white fluorescents) for vegetative growth and more reddish (HighPressure sodium, warm fluoro's) light for flowering. Though it'spossible to grow great buds under fluorescent lighting and a few will even argue their superiority to HID's, most indoor growers use High Intensity Discharge lights such as MH and HPS, and many use fluoro's for vegetative growth and HPS for flowering. It's very important to have the light as physically close to the canopy as possible without burning the foliage and still allowing for even coverage.Many new growers believe that "Droppin the light" closer to the plant will be beneficial. Besides heat stress, the bulb puts out radiant energy that causes leaf burn (Note it is possible to complete a grow using just HPSor MH)
Genetics. Its an easily overlooked factor. Some strains simply have the potential to yield more than others. Having a heavy-yielding strain doesn't automatically equal big yields either. It only means that the potential for heavy yields is there. The grower must provide the optimum environment for that particular strain inorder for it to be able to reach it's yield potential, and each strain has slightly unique requirements. Also, within a strain there are usually several phenotypes, each of which will exhibit unique characteristics which is to say that some pheno's of a particular strain will weigh more than others.
Plant/root/container size. Obviously, the longer a plant is veg'd, the bigger it will get and the more it will yield. Almost always overlooked because they're unseen are the roots. Root mass is directly related to bud production. Simply put, the more roots you have the more bud you will (potentially) have. Be sure to always allow plenty of space for the roots to grow and spread out, even more-so in soil. A general rule of thumb is 1 gallon of soil for every foot of plant height.
System. Scrog (Screen of Green), Sog (Sea of Green), Vertical gardens. These systems have a higher yield than comparable large plant system over the same time period, but are slightly more complex to implement.
Grower's skill. Growers can add yield by: using additives (like B1, kelp, enzymes), foliar feeding, and topping (FIM).
In addition. Tricks like keeping nutrients and the air tempswarm during night cycle can help final yield. Although it's a topic ofhot debate, it's generally thought that any system that supplies theroots with maximum oxygen (aeroponics) would outperform a system that restricts 02 input such as (soil). This is a generalization however, and the increased workload in effectively implementing more complex systems may or may not be worth it for you.
So as you can see, there's much more to yield than throwing someplants under tons of light with tons of nutes. Before one becomes too concerned with yield, one must first learn how to grow plants well, learn how to "listen" to the plants and give them just what they need. It's best to start with simpler methods, in fact, I think the simpler method is always the better one. Learn how to grow strong, healthy, fast-growing plants and the yields will come.