K.P.
I haven't tried any of these, but these come from the "Talking Dirty Laundry with the Queen of Clean" book by Linda Cobb. She says to pick one and try it, but don't machine dry it until the stain is removed.
1. Sponge on rubbing alcohol, repeating several times. If the stain persists, sponge with white vinegar and rinse. Work in your favorite laundry detergent and rinse well.
2. Rub white nongel toothpaste into grass stains. Rub well, then rinse and wash as usual.
3. For jeans, apply undiluted alcohol to the area and allow to soak 15 minutes before laundering as usual.
4. Zout and Spot Shot Carpet Stain remover also work very well on grass stains. Follow the label directions.
Some general things to try:
Clean the washing machine... fill the washer with hot water. Add 1 qt. of chlorine bleach. Run the washer through the longest wash cycle. When the washer is still wet--add 1 qt. of white vinegar and run the washer through the same cycle again. Do this every 3-6 months.