The temptation to overfeed is almost is strong as the urge to overwater. But overfeeding makes the plant weak and susceptible to disease. As with water, plants that get a lot of light need more fertilizer than those in dimmer sites. And unless a plant is actively growing in winter, don't feed it all during the dormant season.
What should you use to fertilize houseplants? Not the blue crystals you mix in water—they're a synthetic that stimulate unhealthy growth. Instead, use a weak dilution of fish and seaweed fertilizer such as Sea Rich from Gardens Alive! in convenient liquid form.
Balanced fertilizers—that is, with an equal ratio of the three main nutrients, nitrogen (N), phosphorus (P) and potassium (K)—are best for houseplants. Frequent weak applications of fertilizer are better than infrequent heavy applications for houseplants.
Potting and repotting
Eventually, all houseplants need to be repotted either because they've grown too big for their pots or because they need to move into fresh potting mix. Generally, you should repot your plants about once every other year. Do it in the spring or summer when the plants are actively growing.
When you remove the plant from its pot, shake away as much of the old soil from the roots as possible. If the plant's rootball is so dense and tangled that the old soil won't shake loose, use a large knife to slice away an inch or two of the rootball on all sides and the bottom. Add fresh soil to the bottom of the pot, then set the root-trimmed plant back in and add fresh soil in the space you created around the sides.
You can use any commercial potting soil for most houseplants (avoid those that have fertilizer blended in). Your houseplants will be healthier and grow much better if you mix compost in with the potting mix. Be aware that certain plants prefer special soil mixes. Cactuses, for example, need extra sand for great drainage. Orchids generally prefer a very loose, bark-based mix. Again, check plant tags for any special needs the plant may have.