1. Use products on your pet wisely
Fleas, ticks, other parasites and certain skin conditions often necessitate the use of shampoos and topical treatments. Not following the usage directions is one of the most common causes of pet toxicity. This is definitely a case where more is NOT better! Please read and understand all directions when using shampoos, dips, spot-on treatments, etc. for your pet. Also observe the "after treatment" before letting children be around and handle the pet to reduce unnecessary exposure.
2. Only use products and medications designed for the pet
Do not use dog products (or medications) on cats and vice versa. The dosing is often different, and things that are OK for one species may be quite toxic to another. The same goes for using human medications and topical treatments on pets -- do not do so unless under direct recommendation/advice from your veterinarian.
3. Keep all medications out of reach
Dogs have been known to down large quantities of pre-natal vitamins, ibuprofen, Vitamin C, and so on. It may be curiosity, it may be the taste (many have a sweet or even chocolate flavored coating). Pet medications may be meat-flavored. Whatever the reason, medication overdose is an emergency. The sooner the better to rid the body of excess toxin. Please call your veterinarian or Animal Poison Control to find the best way to rid or deactivate the toxin (via vomition or activated charcoal).
Read more about the most common poisonings and how to prevent them: