If only it only took just one thing to make your plants burst with beauty, right? There are of course lots of tricks, but one of the biggest problems with growing plants in containers is how quickly they dry out. Plants that continually get too dry become stressed, stunted, drop leaves and don't grow as many flowers or vegetables. Most of the commercially-available potting mixes include mostly peat moss, which is not a very good soil medium for growing plants. It is hard to soak when watering and dries out quickly. In particularly hot and dry weather, it will feel like you are constantly trying to keep up.
Compost is the answer to your gardening woes. Compost will add enough fertilizer that nothing else should be needed. But more importantly, compost holds water. When you water your plants, the compost will suck it up easily and release it slowly. Potting mixes with mainly peat tend to just let the water flow out of the pot rather than capturing and retaining it. Though it can get saturated, usually once it gets dry, you have to go back two or three times adding a little water each time to get the peat to start soaking it up.
If you are starting from scratch with filling your pots, get a bag of compost along with the potting soil. Mix 1/2 and 1/2 of each, stir until it's uniform in consistency and use this to fill your pots.
But even if you are starting from a ready-grown pot of plants and don't want to dig the whole thing up, compost will still help. Add an inch or two on top of the existing soil, work it in a bit with a trowel and water well. The compost will act as a mulch and keep the pots from drying out nearly as quickly.
Giving your plants a big drink in the morning before the sun gets hot also will help keep them happy throughout out the day and reduce plant stress. Compost can make the difference from pots of plants that seem impossible to keep watered to fairly care-free potted plants that don't need as much babying.