Handwashing and proper disposal of tissues are the best things you can do. You could also start taking ecinacea to boost your own immune system. The best thing you can do of course is to breastfeed your little one and protect them that way. If baby does get a cold then breastmilk or saline squirted down the nose before a feed helps. My 2nd son got a nasty cold at 7 days old and whilst it was not fun, we got through it and he seemed to have a very strong immune system after that.