rIf you begin to notice container plants not looking their best, it is likely time to repot them. Some plants need to be moved up to a larger container, some can be replanted in the same container. First, take the plant out of the pot (I know duh). Inspect the roots for problems (rot or root bound). If the plant is root bound gentle tease the roots apart. A little root pruning will not harm a potted plant. If it appears the rots have begun to rot, it is likely that the plant is over watered or the container does not adequately drain. This may smell bad. Check the drainage and make sure to allow the plant to get to the damp side of dry before watering again. If the plant is too large for the original container, repot it one size up. If it is a good size for its pot, return it to the original container. I like my plants to be level with the lip of the container. I add enough potting mix to raise the plant to about 1/4 below the lip level, place the plant, and add more mix around the plant and just over the root zone. Don't plant the too deep. The plant should begine to recover soon. Container plants may be mulched just like bedding plants. Some choices are cedar mulch, colored wood mulch, expanded shale, and washed rock. Mulching helps retain water and prevent soil from being washed from the pot during watering.
For a quick fix to refresh the containers simply add a layer of the potting mix to the top of the container and water with fish emulsion or other liquid fertilizer.
I like to customize my own potting mix. To make one gallon of potting mix use two parts soil less mix (the cheap stuff) and one part screened compost. To this add 1/4 cup bone meal and 1/4 cottonseed meal or blood meal. Watch out for cats if you use the blood meal. I also like to water the plants once a week with fish emulsion. This smells bad but works great (again watch out for cats) There are plenty of good commercial products that allow you to skip these steps.