Help! I found spider mites!
Have you found some delicate webbing -- usually at corners of leaves, or where the leaf meets the petiole? Do you see tiny white, red, or black dots on the undersides of your leaves that shouldn't be there?
Yeah, it's spider mites. They're annoying, but you can very likely save the plant.
Here's what to do.
Standard, tried-and-true:
• Put a little bit of dish soap in a spray bottle, then fill with water. Douse the plant in it, making sure to coat the entirety of each leaf. The dish soap degrades their exoskeleton on contact.
• Rinse off the diluted dish soap, again making sure to get every nook and cranny. Soap will also degrade the cuticle of the leaves over time (fairly quickly) if left on. You don't need to wait before rinsing, as some will tell you to do.
• Immediately place your plant in front of a fan on high, and leave it there for as long as is feasible, especially for some plants with sensitive leaves, such as African violets or String of Pearls/String of Hearts. SOP/SOH in particular are extremely prone to leaf rot when there's standing water on the leaves; sitting it in front of the fan will evaporate the standing water and minimize the likelihood of losing any pearls/hearts to rot!
• Check EVERY OTHER PLANT you own, or at LEAST every other plant in the same room. They travel terrifyingly well, and can reestablish infestations from a single individual. They're so annoying.
• Keep any infested plants in quarantine, if at all possible -- ideally, a completely separate room from the rest of your collection, but somewhere with adequate lighting (often this means a grow lamp). Check each quarantine plant daily for evidence of mites returning; re-treat with diluted dish soap if you see any hint, even if it's small. As a rule of thumb, keep each plant in quarantine until you haven't seen any pests on it for two weeks (some people will only move either all or none of the quarantine plants back out to gen pop at a time, which is likely a good idea).
Optional/alternate method:
• If you live somewhere where it's available, get Bonide systemic; it's not available in Canada, so I unfortunately can't guide you as to its use.
Things NOT to do:
• Panic! Things are going to be fine.
• Leave the soap solution on the plant overnight/without rinsing at all -- this will harm the leaves
• Buy specially-made insecticidal soap (the huge, nationally-known greenhouse I worked for uses Dawn dish soap unless an infestation is bad enough to warrant heavy-duty insecticides. If it's good enough for them, it's good enough for me, and insecticidal soap feels like a money grab.)
• Ignore the problem any longer than you have to!