Following from the previous post.
Most indoor pests are insects or arachnids and can be controlled by the many insecticidal soaps on the market now. Actually, ordinary soapy water will do, but you must be sure to use soap and not household detergent. Soap (technically itself a type of detergent) and other detergents have different chemical activities and different effects. To confuse matters, sometimes soaps are marketed as detergent and vice versa. Dishwashing liquids, shampoos, some liquid hand soaps, and most modern laundry detergents/soaps are detergents. Bar soap is soap, and the old-fashioned washing crystals are soaps as well.
Soaps work by dissolving the waxy coating that insects secrete to prevent water loss through their exoskeletons, so they dehydrate. Detergents don’t do that. However, detergents are better surfactants, so a bit added to soapy water can help the liquid to coat the plant. I suppose a detergent solution might work by engulfing and suffocating the insect, but don’t depend on it.
If you make your own soap solution, beware of making it too strong or it can burn the plant. If you’re worried about this, test it first on a single leaf or small section of the plant (leave it on for a couple days to see if the leaf goes brown and/or shriveled). It should, however, be strong enough that the water feels soapy to the touch.
Do not use soap on ferns, cacti/succulents, or conifers.
Garlic spray, made by mashing garlic cloves and soaking them in oil for a few days, then mixing this with water (yes, I know oil and water don’t mix) and a little detergent, works very well on some pests, but not others. Test for damage to the plant first and dilute if necessary.
Spraying your plants indoors can be a messy job, so you might want to put the plant in the shower or bathtub before you start. Be sure to spray every part of the plant: under the leaves, in the cracks and joints of the stems, all the little crevices between the flowers – everything. Spray until the point of drenching; you want to see the stuff dripping off. You will probably need to do this several times at intervals of two or three days. It’s not a bad idea to wash the plant off the next day by spraying with clean water, but it isn’t usually necessary.
Another way depends on identifying the exact species of pest you have. If you can do this, you might be able to order special sticky traps. These traps are loaded with pheromones that attract the (adult) pests, which get stuck. Since these traps are usually hung on or around the plants, they are generally only available for pests that fly at some point (I could be wrong about this; haven’t looked into them for a while).