Like Joy said, I have found that baby wipes are the best for that! I didn't want to use anything harsh, especially on carseat straps. Always worked for me!
Thanks Joy - I will try them!
Spit up is one of the hardest things to get out!
I have found the when spit up is still wet you can use baby wipes to clean it up. So even though it's old and dried, try some baby wipes.