1. The Wetter The Better (The wetter the clay is, the easier it is to join.)

2. Join Like To Like (The pieces being joined should be the same dampness.)

Why These Rules?

1. The wetter the better: The more water between the clay particles, the more they can slide around and intermingle when clay pieces are next to each other, and the more soft and plastic the clay is, making it easier to physically blend clay pieces into one another.

2. Join like to like: When joining clay parts together, they should be equally wet (have the same moisture content). As clay dries it shrinks - physical water between the clay particles evaporates, and the clay particles draw nearer to each other. If a wet clay piece is joined to a drier clay piece, the wet clay piece has more shrinking left to do than the drier piece, and so (as it does shrink more) it will create stress and want to crack away from the drier piece. Unknowing beginners may be able to join a wet piece of clay to a near bone-dry piece (typically something they have been working on for days and have let dry out too much), but in the end, no matter what they do, the wet clay will shrink more than the drier clay and all their effort will usually go for naught. The simple solution is to spend a little effort before attaching things to re-wet the drier piece, and/or dry out the wetter piece, until the parts are evenly wet, then join them. 

Paying attention to this rule will make working with clay 100% less frustrating.