Rings: Visit your local jeweler, they will size you for free. If you frequently buy rings, consider getting a finger sizer.

If you copy and paste this link into your browser, it will show you an example of a good, economy finger sizer. vvv

Chains and Necklaces: Use a string or a shoelace to measure where you'd like the chain to sit on your neck/chest.

Bracelets and Anklets: Use a string or a shoelace to measure your wrist or ankle where the jewelry is meant to go. Make sure you account for how loose/tight you want your jewelry to fit.