How do you potty train a 3-5-month-old puppy (Boxer)? Please send me tips or an (free) on-line link.?

We got our puppy (mainly I wanted her) when she was two months old. She's now four, and showing absolutely no signs of potty training, much to my parents' dismay. I really don't want to get rid of her, but we've tried everything! We take her to a consistent spot, reward and praise her when she eliminates, and thoroughly clean up her messes in the kitchen. She doesn't have a medical disorder, we got that checked out. Please, for her sake, HELP!!!