Yeast is available in dry or liquid forms. Most professional brewers will use liquid yeast, and usually cultivate and store the same strains for use over and over again. The yeast that falls to the bottom of the fermenter once its work is done (flocculation) is not "spent", but merely dormant, awaiting a fresh meal of wort sugars that will restart the fermentation process.
To keep yeast viable for future use, brewers can store it under beer or water (which is becoming more common), although its potency will drop off over time unless the brewer is skilled and resourceful. Because it is such a picky creature to keep happy and potent, Jason Meyer of Driftwood Brewery advises "use pure culture liquid yeast only, and pitch lots of it!" Dry strains of yeast may be stored for much longer, but the results are not held in as high regard:
Liquid yeast. Yeast can account for 40% of the flavour compounds. Pick the right yeast for the style. (in the last 15 years, dry yeast has come a long way, although there is still limited variety)
Terry Schoffer (Cannery Brewing):
For increased flavor and body always use liquid yeast. Most micro brewery’s will welcome home brewers and give them yeast and sometimes tips for free. Show your appreciation and bring them a sample of your home brew or buy something from their gift shop.For those who must or want to use dry yeast, Terry Schoffer flags the importance of rehydrating the yeast properly before use, "make sure you re-hydrate it by heating 1 cup of water to 40C, adding your dry yeast and letting it stand for fifteen minutes before pitching. This will promote healthy yeast growth and keep the nasty bacteria count down. Never use wort to hydrate your yeast."
Dry yeast can produce decent results. However, BA member Homebrew42 warns us about homebrew kits that come with packets of "generic 'ale yeast' that is typically of low quality. You're never going to brew a fantastic English bitter with an old, stale packet of characterless 'ale yeast'."
Overall, the message seems to be if you take the time to understand what yeast is and how it works, and get in touch with local beer makers who are experienced in handling yeast, your beer will turn out better. And you might even score some free yeast in the process.