While I knew a lot of parents who swore that changing diets dramatically helped their autistic kids (removing sugar, gluten, and artificial ingredients, etc.), I never did that with my son. He was a finicky eater, and I didn't want to limit his food choices further. Plus, his pediatrician advised against it, believing there wasn't a need.
However, when my son was diagnosed at 4 with Sensory Integration Dysfunction (which often goes hand-in-hand with autism but not always), he started weekly sessions with an occupational therapist who immediately put him on a “sensory diet.” A sensory diet doesn't involve food but activities that are designed especially to meet the needs of a particular child with sensory issues.
My son, for instance, was weak and floppy with low muscle tone. He couldn't peddle a tricycle on the track at preschool, pump a swing, or climb a tree. His fine motor skills were poor, so he couldn't color without discomfort or hold scissors correctly. He was hyper-sensitive to touch. He didn't like the feel of sand on his skin, so he avoided the preschool's sandbox. He was irritated by the tags on his shirts, so I cut them off. He screamed bloody murder whenever someone tried to cut his hair, so we stayed away from barbers.
His occupational therapist came up with a whole “menu” of activities to help my son lead a life where he wasn't so agitated, afraid, and unable to participate. She did them with him at our weekly sessions while I watched, and then I continued at home. To help him become more tolerant of tactile experiences, she had him drive Hot Wheels through mounds of shaving cream, make drawings in chocolate pudding, and create sculptures with play-dough. To strengthen his hand and finger muscles, she had him pull duct tape off the floor, pick up cotton balls with tweezers and chopsticks, peel and place stickers, and play games such as "Don't Break the Ice," "Operation," and "Don't Spill the Beans." To help with his vestibular system (movement and balance), she had him jump on a mini-trampoline, pull a wagon, walk on a balance beam, and rock in a rocking chair.
Without a doubt, having a caring and competent occupational therapist was the most important thing I did to help my son (a fantastic speech therapist was another asset). My pediatrician's knowledge of autism was limited (I'm sure it's grown since then), but my son's OT was a wealth of information.
Because she had a teenage son with Asperger's, we connected on an emotional mom-to-mom level as well. Going to a support group with parents of autistic kids was not at all helpful to me (just stressful) since autism is a spectrum disorder and each child is affected differently. I believe parents should work with a professional one-on-one and be wary of the misinformation on the internet. Parents of autistic kids are desperate for cures (I know I was), but autism isn't something that can be fixed. Our children can improve greatly, though, with early intervention services and a sensory diet. "The Out-of-Sync Child Has Fun: Activities for Kids with Sensory Integration Dysfunction" by Carol Stock Kranowitz was an invaluable resource for me.