Congrats on your baby, and on being a wonderful breastfeeding mom. Depending on how frequent the spitup is, I'd recommend different strategies:
1. Occasional/periodic spitup: This is 100% normal; you don't need to do anything differently.
2. Frequent spitup: I agree with Cheryl's suggestion: try avoiding things like broccoli, cauliflower, onion, cabbage, cukes, and peppers.
3. Constant spitup, plus symptoms like arching back and sort of roaring, diarrhea, bloody or mucusy stools: These are symptoms of more serious dietary allergies or intolerances. You may eliminate dairy, nuts, eggs, soy, and/or wheat from you diet -- dairy is the most common culprit in infants.
I highly recommend the website Kellymom for any and all breastfeeding questions/challenges. And of course, if the spitup is constant, or if you notice any stool abnormalities, check with your doctor.
Best wishes,
Mira