Lots of hand washing of course and the usual type of activities that prevent cross contamination like not touching your nose, mouth or eyes while your baby is sick unless you've just washed your hands. Also, taking the right vitamins and supplements can help enormously as well as adding the right things to your diet.
For vitamins and supplements take at least one per day of the following to boost your immune system:
The mineral Zinc
For food additives to keep your body strong against infections:
fresh garlic (anti-bacterial properties)
curry powder (super-antioxidant - not to mention delicious!)
Plenty of protein
real cheeses (anti-bacterial/antibiotic properties)
Also, drink plenty of water to keep your system flushed of all of the negatives and remember to brush your teeth at least twice per day and rinse with a good germ killing mouthwash (don't forget to floss!). Your mouth is the first line of defense and is usually the first point of entry for germs. Remember to keep it clean and it will help enormously. If you want a cheaper alternative to mouthwash, you can do a good warm salt-water rinse a couple times a day.
That's all I can think of off the top of my head. I hope you get some great responses!