Rotisserie chickens are almost invariably brined in a mixture of water and salt. That plumps them up but dramatically raises the sodium content. Which is why they taste so good. People seem to like the Costco birds best; I like BJs (don't go there). Giant and especially Safeway sell chickens the size of Cornish game hens and they don't even taste very good.
You can also brine your own chicken. It makes it much more juicy. This is the best recipe I've found for simple baked boneless, skinless chicken breasts. Incredibly quick and easy.