I don't have any personal experience with heroin withdrawals, but I know a few things that help with opiate w/ds in general
Gastrointestinal symptoms:
Imodium AD (similar in structure to Demerol)
Pepto Bismol, Rolaids, and Tums for stomach upset
Ginger root tea/peppermint tea (a miracle for nausea)
Cannabis for nausea, vomiting, and upset stomach
A bland diet with plenty of water
Pain:
Acetominophen (Tylenol)
NSAIDs (Ibuprofen, Naproxen, and Aspirin)
Heat: hot baths, a heating pad, etc.
Muscle relaxers (Flexeril, Skelaxin, etc.) Prescription only*
Cannabis (also great for certain types of pain!)
Calcium and Magnesium for muscle spasm
Insomnia:
Antihistamines (Benadryl, Tavist, Chlorpheniramine, etc.)
OTC sleeping meds (Doxylamine Succinate, Unisom, Sominex)
Nyquil
Herbs like Chamomile, Valerian root, Passion Flower, Hops, and Catnip, which have sedative properties.
Benzodiazepines (Xanax, Valium, Ativan) Prescription only*
For cravings, there's not really much you can get OTC that will take your mind off the drug, but certain psychedelic drugs like Ibogaine have been used in treatment of the psychological part of opiate addiction, and marijuana has helped some recover by easing cravings. The same is true for psilocybin, mescaline, and ayahuasca, all of which are largely illegal. One legal alternative is DXM, which is available OTC in most countries...