Washing your hands
You've known that hands need to be washed after using the bathroom, and before dinner, since you were potty trained. But how many people really wash their hands every time they touch a piece of raw meat, ride a public bus or train, or stroked your pet (or someone else's)? Most of the time, we don't think about these things at all or simply assume that our immune system will take care of any bacteria you encounter. When you are pregnant, a little more caution pays off. Wash your hands when you've been gardening, have handled meat or vegetables (because of listeria, toxoplasmosis, and other things you really don't want), have been on public transport or in someone's car, or have cared for other people's children especially in a group environment. Diapers and a bathroom visit also require hand washing, obviously.
Stay away from dubious foods
Raw meats and unpasteurized milk and dairy are a potential source of listeria and other bacterial infections. Raw eggs (found in homemade mayonnaise and ice cream, for instance) pose a huge salmonella risk. Buying food from unregulated food stalls on the street, or dubious-looking fast food outlets, is never recommended during pregnancy. When you cook at home, you should make sure that your meat is well done, even in the middle. For more info also see: Listeria infection in pregnancy.
Avoid people who are ill
You may not normally think anything of going on a sick visit, or just hanging around people who have colds. When you are pregnant, being a germophobe makes sense. While you are at it, try not to share glasses, cutlery or plates with anyone, and young kids in particular. Don't worry about being rude; everyone will understand that you want to take care of your unborn baby's health.
The cat litter tray
The cat litter tray has become a classic almost everyone who has ever been pregnant or has just thought about it knows that cat feces can contain toxoplasmosis and that this is really dangerous for unborn babies (doubly so if mom has not developed antibodies from previous contact). Even though The Cat Litter Tray is so famous I feel silly even mentioning it, it's got to be on the list. The cat litter tray isn't all that dangerous in itself, as long as you wash your hands right after. The same goes for working in the garden, where cat poop is usually abundant.
Let's talk about sex(ually transmitted diseases)
STDs are hardly a nice topic, and mention sexually transmitted diseases in combination with pregnancy and you've already got a taboo on your hand. Yet, there is never a more important time to get tested for sexually transmitted diseases than during pregnancy. The need to get tested exists even if you have been monogamous for a long time (or even all your life). If you don't have an STD, you can have a peaceful pregnancy. And if you do, you can get timely treatment. Some STDs induce miscarriage, others can lead to an ill baby, and yet others affect your whole life (think HIV/AIDS). Just get the test, already!
Are you up to date on your vaccines?
Have you had a tetanus shot within the last 10 years? Do you need a flu shot? And... are you immune to vaccine-preventable diseases like measles and mumps, which can be dangerous to a fetus? You can have a flu shot during pregnancy, but most vaccines are not suitable for pregnant women and should be administered several months before you start trying to conceive. If you're already pregnant, you can still talk to your doctor about preventing these diseases.