Annuals
The first thing to consider when watering is how much rain we've had and what's in store for the next day or two. If it's been a rainy season, be careful not to give supplemental water too often, or you will likely cause root rot in the plants (because of the standing water in the drenched soil). By the same token, during a drought, you'll have to provide all the water your plants need since Mother Nature isn't quite doing the job.
If your annuals are in a container or if they are newly planted, try to check on them twice a day to ensure they don't dry out in the summer sun and heat. For most people, this means a quick check and water in the morning before work and one when they come home. In the morning, err on the side of overwatering, which will help the plants stay hydrated through the hottest part of the day. In the late afternoon or early evening, err on the side of underwatering so they don't stay too wet during the night when they need less water.
Annuals that are planted in beds and have been established for at least a couple weeks require much less maintenance, and as long as they are in good soil with the proper sun conditions. Established annuals probably won't need additional water from you except on the hottest days. Just keep an eye out for wilting and adjust your watering accordingly.
Keep in mind that succulents and shade perennials require much less water than others, so plants like portulaca and fuschia will appreciate drier soil. Scaevola is just the opposite - it's nearly impossible to overwater this plant because it dries out so quickly, so go to town with the water in these hanging baskets!
Perennials
Newly planted perennials should be checked and watered as needed every day for at least a week unless it's been rainy. Be sure that you have planted your perennials in the proper sun conditions; for example, shade plants planted in full sun will have much greater water requirements and will likely not survive. Most plants require moist, well-drained soil, but some have special needs, so always be sure to read the tags to determine exactly what they'll need once planted. If after a month in the ground your perennials are still drying out without regular watering on your part, or if they're constantly soaked even without additional watering, your soil is probably not sufficient to maintain them and you may have to relocate them.
Sandy soil that retains too little water can be beefed up with potting soil, garden soil, peat moss or topsoil, whereas very moist soil can be given more drainage by using layers of processed stone or sand in the bottom of the planting hole. If these fixes don't help, you may consider changing the location of the perennials in question.
Succulents like sedum (stonecrop) and hens & chicks require much less water than most perennials. Be sure not to plant these in the same area as plants that require lots of water, because it will be too difficult to create the proper environment for these polar opposites. One way to keep succulents a little drier is to skip the mulch. Mulch is fantastic for plants that dry out quickly because it helps retain the water in the soil - the same reason it's not a great bet for succulents. Hens & chicks, in particular, look fantastic against crushed stone, which is a much better bet if you're looking to decorate and finish the look of your garden without mulch.
Trees & Shrubs
For the first week or two after you've planted a new tree or shrub, water it generously to help it establish itself and start stretching it roots. The larger the shrub or tree, the more carefully you'll need to check it and make sure it does not become too dry. With most trees & shrubs you can see the leaves drying out at the very top of the plant - this is the first sign that you need to water it right away. Left too dry for too long, the outermost branch tips of many woody ornamentals will not recover.
When watering trees & shrubs, in particular, it's better to water very thoroughly and less often than superficially many times a day. Ensuring that the water soaks deep into the ground will help the tree or shrub root deeply into the ground. Superficial watering will cause the roots to stay close to the surface in order to take in the necessary water, and this will keep the plant from becoming healthy and established. When in doubt, take your finger and gently push one of the leaves up or down against its natural hanging position - a well watered plant's leaves will resist your nudge and spring back to their natural position on the stem or branch. A dry leaf will simply hang there where you left it.
For evergreens, including groundcovers and hedges, be sure to water extremely thoroughly, especially if you start to see any browning of the needles. This browning is called "die back" and it is very difficult to get the plant looking healthy again after it has set in. So keep the hose handy if you start noticing the color change.
Dogwoods and Harry Lauder Walking Sticks also require a little extra thought on the gardener's part. These trees & shrubs naturally have drooping or wrinkled leaves, so don't mistake this distinctive look for wilting!