Once you've planted your garden, it needs water more than anything. If too little water is provided, your plants won't be able to develop properly, and they're more prone to pest and other problems.  Of course, too much water can be equally harmful, as it can lead to rot and other problems.

More often than not, sick plants are most in need of water, not chemicals. Correct watering would save the majority of plants lost in gardens.

Usually (unless you live in very rainy places), you need to help Mother Nature, because natural rain water won't always do it. The best way to notice when your garden needs water is to eyeball it. If the plantbed's dry to a depth of a half inch, you need to water (and how often depends on temperature). With highs in 70 range watering once or twice weekly will usually suffice -- when higher, you'll need it more often (every other day, or more).

When watering you want to make sure the soil is well moistened and kept damp. This can seem to take forever, but you'll be beter off watering completely once per week than dampening everything every day (but not thoroughly). Hose nozzles are normally too forceful -- instead, try to use your thumb or a nozzle that allows a good flow, but not overwhelming.  Aim for the base of the plants (where the stem meets the soil, of course), and you can wet the leaves as well. Mornings are the best time to water, and night/really late evening's the worst time to water as damp foliage at night can lead to molds and fungi.

Irrigation systems can be awesome if you need help -- drip hoses are best as they minimize evaporation and put water right at the base of the plants. Also, sprinkler systems are more prone to mechanical failure, and they're often difficult to detect (since they run early in the morning, when you're asleep!).

Photo courtesy of a2gemma