Below is an abstract from an ophthalomolgy publication on the most common contact lens related eye infection Microbial keratitis.
"Risk factors in cases of moderate to severe microbial keratitis for daily contact lens users included occasional overnight use, poor storage case hygiene, smoking, solution type and infrequent storage case replacement. Contact lens material, age of wearer and gender were not associated with moderate to severe microbial keratitis.
The study found that patients paying proper attention to contact lens case hygiene could eliminate 50% of moderate to severe cases of microbial keratitis. Slightly more than a quarter of those incidents could also be prevented by contact lens case replacement.
In short, replace your contact lenses when they should be replaced, clean your storage case thouroughly every time you put your lenses on and replace your storage case every three months.