|inspiration from here|
I pretty much follow the rules when it comes to container planting and always ensure my pot has all 3 elements - an upright plant, a mounding plant, and a trailing plant. I usually like to have 2 different types of trailing plants just too add a little bit of interest to the outside of the pot. If you look at the tag on the plants before purchasing, it will usually indicate which of the 3 the plants is so you can plan accordingly. My pots always look a little sparse when I first plant them but trust me when I tell you that they fill in and if the plants are too large when you plant them - they tend to get overcrowded rather quickly. I use the moisture advantage potting soil that is designed to provide a better balance of moisture in the soil so that if you miss a watering or 2 - you don't lose all of your plants. I used it last summer and it seemed to work pretty well so when I changed out the soil in my pots this year - I replaced it with more of the same. After the plants have been potted for about a week, I like to add mulch to the top of the plant to cover the soil and provide my plants with a little insulation from the hot summer sun. I've found it works best for me to arrange my pots in the beginning to mid-May and they usually last me until the weather cools in September. Generally speaking, it takes me no longer than 10-15 minutes to arrange the pots and get them planted and by selecting your own flowers you can definitely save some cash over selecting the already arranged containers at the store.
Looking for more inspiration - you can go here for some other flower pot arrangement suggestions, courtesy of none other than Better Homes and Gardens!