I hate to admit it, but I do use a water-soluble chemical fertilizer. I never use them on my other gardens because I use so much compost and other organic fertilizers. The problem with pots, is that every time you water them, some nutrients wash out. I use the fertilizer at one-half strength about once a week, and all my flowers do great. When you are planning your container, choose some plants for each pot that will trail or cascade over the side.
Some good ones are ivy geraniums, lobelia, allysum and some petunias.
I like to combine flowers and vegetables. Red looseleaf lettuce looks good with almost any flower. I also try to put parsley in every container. I've had the best luck with cherry tomatoes, like Sweet 100. Make sure you put the cage in right away, or they will soon be sprawling all over your deck or patio.