When measuring a wrist ( or ankle ) measure across the widest part of the bone. You will want a close measurement, but do not make the measurement skin tight. You should have enough room to get one of your fingers under it. All jewelry is strung on a flexible wire, if it is too tight or stressed too much the wire will break.
Skin contains oil, and over time your jewelry (especially Sterling Silver) may loose it's luster (shine and may become black and tarnished looking ) It is important to remember to polish your jewelry from time to time. This can be done with a polishing cloth. Make sure to get a polishing cloth made specifically for your jewelry-- silver cloth for silver jewelry, gold cloth for gold jewelry. Wal-Mart carries polishing cloths in their Jewelry Department.
After you have your cloth hold your jewelry in one hand, the cloth in the other. Carefully rub each bead with the cloth making sure to get between the beads. Continue this process for the entire piece of jewelry-- don't forget to polish the clasp also. This is a time-consuming project, but well worth the effort. 
Always remove your jewelry when using cleaners, they may dull or pit your beads/stones.
Never wear your sterling or stone jewelry while in a swimming pool. The chemicals in the water will ruin your jewelry.
Always remove your jewelry that contains, pearls, opals, turquoise, before using soap & water (washing dishes, bathing, etc). Soap causes pearls and opals to lose their luster (shine) and turquoise is a porous stone that may absorb water & soap, and this could ruin your stone.