When I was in a pinch recently, I grabbed a bottle of Antibacterial Hand Sanitizer and applied it to a milk spill that was a few days old.  To my surprise, it cleaned it and got rid of the milky smell.  After that I went crazy and started using the sanitizer on all the spills/ground in animal crackers/etc. that we had on our carseats and jogging stroller.  It works incredibly well!  So, when you don't feel like taking the carseat cover off and the stroller is too much of a pain to wash, spot clean with some antibacterial (purell, etc).