emerelle2k
It's difficult to answer not knowing exactly what it is you spilled on it. If it was oily at all, try rubbing some dish washing liquid on it and washing normally.

StevenQ
Try soaking it in Simple Green for a day or two, then wash as normal. If you can, just soak the part that has the spots. I've got paint to come off of jeans with that method.

geedee
Best thing to do is soak the stained area for a few hours at least...since you already washed and probably dried it. After you allow the water to penetrate the fabric have a tooth brush and deluded dish washing liquid ready and scrub the stained area twice for about 5 minutes, then rinse it out and check and see if you might have to scrub it again, a little baking soda might help. Works for me.....

M M T
Things for you to try: (1) I liked the answer about Simple Green but it's not necessary to soak for a couple of days. Just spray the stains, let the spray on there for about an hour and wash normally. (2) The dish detergent might work if it's a grease fighting detergent but you have to be careful and not use anything that might contain bleach. (3) I have good luck with Spray & Wash on a lot of stains. The key is to spray the stain and leave the stain remover work for an hour or so before you wash. You can't spray and just chuck it in the washer and expect good results.