The third factor for a good fermentation is temperature. Yeast are greatly affected by temperature; too cold and they go dormant, too hot (more than 10°F above the nominal range) and they indulge in an orgy of fermentation that often cannot be cleaned up by conditioning. High temperatures encourage the production of fusel alcohols - heavier alcohols that can have harsh solvent-like flavors. Many of these fusels esterify during secondary fermentation, but in large amounts these esters can dominate the beer's flavor. Excessively banana-tasting beers are one example of high esters due to high temperature fermentation.
High temperatures can also lead to excessive levels of diacetyl. A common mistake that homebrewers make is pitching the yeast when the wort has not been chilled enough, and is still relatively warm. If the wort is, e.g. 90F, when the yeast is pitched and slowly cools to room temperature during primary fermentation, more diacetyl will be produced in the early stages than the yeast can reabsorb during the secondary stage. Furthermore, primary fermentation is an exothermic process. The internal temperature of the fermentor can be as much as 10F above ambient conditions, just due to yeast activity. Thus it is very important to keep the fermentor in the proper temperature range; so that with a normal vigorous fermentation, the beer turns out as intended, even if it was warmer than the surroundings.
Brewing in the summertime is a definite problem if you don't have a way to keep the fermentor cool. My friend Scott showed me a neat trick though, he would immerse (not completely) his fermentors in a spare bathtup during the summer. The water in the tub was slow to warm during the day even though temperatures would be in the 90's, and at night the water would be slow to cool, even when the temperature dropped to 45 F. In this way he was able to moderate his fermentation temperature between 60-70 F, and the beer turned out great. I have used this method myself with wash tubs and had great success. You can periodically add ice to the water to help keep it in the right temperature range for your beer. If you don't have a way to keep the fermentater cool, and the beer temperature gets above 70-75F (21-24C), the beer will probably suffer from off-flavors, such as very fruity, phenolic or solvent-like flavors and aromas. One of the best ways to control the fermentation temperature is to buy a used refridgerator and plug it into an external temperature controller. After sanitation, fermentation tempeature control is the most important factor for producing a good fermenation and a good beer.