Summarize the following:
You need to use a large enough pot with good drainage. You might also want to consider pots made out of plastic or resin, since these won't absorb water that should go to the plant. Potted plants need a more nutrient rich soil than garden plants, since they don't get the benefit of water circulation like regular plants. Use a high-quality potting soil and pair it with some compost. Potted plants, like garden plants, are prone to pests and mold. Even more so because they lack the natural environmental counterbalances. If mold develops on your soil, remove the top layer of soil and replace it. If you end up with a pest problem, deal with it promptly before your flowers are negatively affected. Different types of flowers need more or less sun. Carefully plan where you will place your pots so that the flowers get the correct amount. You'll have to keep the light angle, time of day, and directionality of the sunlight in mind when placing the pot (don't just put a sun-needy flower in front of just any window). If you don't have access to enough sunlight, trying using a fluorescent light. If the plant doesn't seem to be taking off in the way you'd like, introduce an appropriate fertilizer for that particular plant. Consult your local nursery for a good product. When the blossoms get wilted or are simply growing in an unnecessary location (such as under the rest of the plant), you can encourage more and healthier blossoms by pinching off the ones you don't want. Check the soil for moisture daily and provide the right level of water to keep the soil damp enough for your particular plant. The temperature of the environment, exposure to natural rainfall, the type of soil, and the type of plant all play a part in how much water your flowers will need. Check your plant every now and again to see if it needs a bigger pot. Turn it over and look at the drainage holes. If lots of roots are poking through, you need a bigger pot.
Use the best possible pot. Use the appropriate soil. Keep an eye out for pests and mold. Give the flowers the right amount of sun. Introduce some fertilizer. Deadhead to encourage more blossoms. Maintain a good watering routine. Increase the pot size if necessary.