Prevention of chafing falls into four categories: staying hydrated, staying dry, using a lubricant, and wearing appropriate clothing.
Hydration: Drink lots of water before, during, and after your walk. This will allow you to perspire freely so the perspiration doesn't dry into salt crystals which will enhance the chafing.
Staying dry: Go light on the deodorant stick as that can make you stickier. Use talcum powder, cornstarch or potato starch to stay dry. If this method doesn't work, it is time to progress to using a lubricant.
Lubrication: Walkers use all kinds of lubricants to keep the skin areas sliding past each other instead of rubbing raw. Good old petroleum jelly is the standby. Apply liberally before your walk. Many walkers also mention Bag Balm and Udder Cream, available at the local feed shop or pet store. Sports stores have newfangled gels that also purport to help prevent chafing. I use BodyGlide, which goes on like a deodorant stick. It is petroleum-free and non-staining. In my research I found many hand creams that advertise their usefulness in chafing prevention. Open up your medicine cabinet and search for something that will keep the area lubricated.
Clothing: Loose clothes may feel good on the trail, but to prevent chafing you need a snug fit. Bike shorts are designed to give a skin tight fit that will prevent chafing and are excellent if your trouble is in the lower body or thigh area. For the upper body, polypropylene or lycra/spandex tops that fit skin tight will do the trick. Nipples are especially prone to chafing, for men and bra-less women. NipGuards or adhesive bandages can provide even more protection than lubrication.
Treatment: Once you are chafed, treat the area like an open wound. Wash and clean with antiseptic (OUCH!) to prevent infection and cover with a sterile gauze pad that allows the area to breathe until it is healed.
Here's wishing that at your next walk you are chafing at the bit (to get on the trail) - not chafing in the 'pits.