Getting a consistent fermentation is a major challenge for homebrewers. The quantity of healthy yeast, along with the fermentation temperature, directly impact the flavor compounds that will be produced along with the alcohol. While sometimes you might want to transfer the beer to the fermentor and let nature work its magic, other times you might want a more direct control over the variables that affect fermentation. If you're already making an appropriate sized yeast starter for your homebrew recipes, the next things to take control of are aeration and fermentation temperature.
At the end of any set of homebrew instructions, there's generally a step that requires you to vigorously shake the wort after it has been cooled. The purpose of shaking the wort is to dissolve enough oxygen into the liquid for the yeast to be able reproduce. Without enough oxygen, the yeast get stressed out and they create undesirable flavors in the form of esters or other compounds. Vigorously shaking the wort works pretty well, but there are a few tools to help keep a closer control over oxygen levels.
While the requirement differs from strain to strain, an oxygen concentration of 8 parts per million (ppm) is considered a pretty good starting point for the most common yeasts. Immediately after boiling, the wort in your brew kettle has a concentration close to 0 ppm, since all the oxygen has been boiled away. Transferring the beer to a fermentation bucket increases that a little bit, but it's still probably not much higher than 1 ppm.
Pure oxygen is the most efficient and sanitary way to aerate your wort. Two pieces of equipment are needed to dissolve pure oxygen. First, a 1.4 oz oxygen cylinder can be found for under $10 at most hardware stores. In my experience, store employees will have a hard time locating this item for you, but they can usually be found by the welding supplies. Each cylinder will last for about 10 batches of beer. The cylinder needs to be paired with a regulator and diffusion stone. This item screws onto the oxygen cylinder, and it can be a little pricey at around $50. In order to use this setup, sanitize the stone and the portion of the hose that will be dipped into the wort. After the wort has cooled, submerse the stone, open the regulator and allow oxygen to bubble through the beer for 60 seconds to get up to 8ppm. If you're brewing an ale that will be above 7% or a lager, give it an extra 30 to 45 seconds. Remove the stone from the beer, rinse off the hose and store it in a safe place, since the diffusion stones can be fragile.
For a somewhat less expensive alternative, and one that doesn't require you to replace the oxygen cylinders, an aquarium tank aerator with diffusion stone is a good option. This will work the same way as the pure oxygen system, but it forces air through the wort instead of pure oxygen. Since air is only 21% oxygen, it will take 15 to 20 minutes to get up to the recommended 8 ppm.
Any of these methods, including shaking, will work best when the wort is cooled to fermentation temperature. At higher temperatures oxygen doesn't like to be dissolved into any liquid, so it's usually a waste of time to aerate wort warmer than 75°F.
Fermentation temperature varies wildly by style. A saison can ferment at 75°F or higher, while a Märzen might be as cool as 45°F. For the most part, the styles most frequently made by homebrewers are ales that should be fermented between 63 and 68°F. During fall and winter it's usually possible to find a cool closet or basement that stays the correct temperature, but the summer brings warmer homes and less-than-ideal fermentation environments.
In addition to higher in-house temperatures, yeast generate heat while growing and producing alcohol. So if the air temperature in the room is 72°F, the actual temperature in the beer can be 77°F or higher. In this range, IPAs, stouts, and most other ales will produce high amounts of fruity esters and burning fusel alcohols.
If you have a lot of extra space for brewing and you already produce lagers using a temperature controlled refrigerator, maintaining correct fermentation temperatures is not an issue. You simply set your fridge for 65°F and forget about it. For everybody else (which is probably most of us), you can use ice, water and evaporation to reduce the temperature as much as 10°F.
I found a huge stack of these $6 18-gallon buckets at Home Depot one day and I knew I hit the jackpot. They're the perfect size to hold a carboy with plenty of room on the sides for water and ice. I fill the bucket about half way full with cold water, slowly submerge the carboy and then add 4 or 5 cooler-style ice packs. I keep another set of ice packs in the freezer, and I switch them out about twice a day. Even if it's 75°F in the closet I ferment in, I can usually keep the water surrounding the carboy to a consistent, fermentation-friendly 65°F.
If the ice water isn't cool enough, take an old t-shirt and place it on the carboy so that the airlock goes through the neck. Then point a small fan directly at the shirt surrounding the carboy. As the shirt soaks up the water and then drys, the ambient temperature will decrease even more due to the evaporation and the magic of physics. With these two techniques, it's possible to produce consistently fermented beer all summer long.