Trees & Shrubs:
Make a small dike around your plant (about a 1/2' away from the trunk) and then fill the dike full with water. 2 - 3 days later check the soil about 1" below the surface; if the soil crumbles between your fingers, fill the dike again; if the soil is sticky leave it for another day or two then check again. Always check the soil before watering :) You don't want to leave your plant totally dry or continually soggy.
Remember to think about the weather when watering. If it has been raining lots you may not need to water as much, if it has been raining just a little the soil may be wet on top but totally dry underneath - always check; if it has been hot and dry or very windy you may need to water every other day instead of every 3. If you keep an eye on your plants they will thrive for you :)
Baskets & Containers:
Stick your finger into the soil up to you first knuckle, if the soil feels dry water the basket or container until you can see the water running out of the bottom of the pot. If the soil is still damp let it sit for another day. Once you have watered the basket or container let it dry out until the dry is crumbly again. Baskets & containers do not like to be continually soggy. Dry is better than wet for baskets and containers; if you maintain even moisture they will bloom for you again and again.