Cleanliness

• Cleanliness is the single most important aspect of home brewing. No matter how high-quality your equipment and ingredients are or how expertly you brew, if your brewing equipment isn't properly cleaned and sterilized, your beer will not turn out well. Brewing is a delicate process; the goal is, essentially, to create an environment in which one type of microorganism (your yeast) will flourish, while keeping out all others. Make sure everything that will touch your wort or beer after the boil, including the bottles and caps, is scrubbed clean and soaked in sanitizer.
  With the drive toward cleanliness, you will want to upgrade at some point to higher-quality materials than most home brewers start out with. Plastic equipment, the standard among new brewers, is cheap and readily available, but scratches easily, making proper cleaning progressively more and more difficult. If you find yourself producing infected beer despite your best efforts, it may help to invest in glass and stainless steel equipment.
  
  

Fresh Ingredients

• Make sure the ingredients you are using are fresh. All-in-one home brewing kits often sit on store shelves for a long time, causing the malt and hops to go stale. Purchase your ingredients instead from a home brewing supply store, whose ingredients are likely to be much fresher. Whenever possible, smell the malt before buying to make sure it hasn't lost its scent, a sign that it has been sitting around too long.
  
  

Preventing Oxidation

• After you pitch the yeast in a batch, do your best to avoid letting any air contact your wort or beer, as the presence of air will oxidize the beer and give it a stale taste. This is often difficult for home brewers, who generally do not have fully-enclosed brewing systems. The best way to go about it, with whatever equipment you have, is simply to plan out each step in the process before you do it, with a focus on minimizing the time your wort or beer will spend in contact with the air. Also, avoid splashing the wort or beer whenever you move it. Ultimately, though it is expensive, buying the equipment necessary for a fully-enclosed brewing process will be the only way to completely protect your beer from being oxidized.