Keep your Master Pure Culture Contamination free. Protect your cultures from dust. Never put a non-sterile inoculating loop or needle into your culture. When you need a sample, just touch your loop to the bacteria. Keep a subculture as your working culture so you do not have to enter your master culture often.
Some species produce spores and survive for extended periods.
Bacillus spores survive for decades. Clostridium spores do not seem to survive as well. Spores of other genera do not seem to live more than a year or so. Some non-sporulating species have survived for long periods. More on this later.