“Some experts think burial is a risky way to try to preserve items.

Nevertheless, the process of choosing items to save in a time capsule can bring people together in positive ways, provide a valuable learning experience and demonstrate ways that items have meaning.” from http://www.mnhs.org/preserve/conservation/timecapsule.php

At the bottom of the web site, there are two good resources: “Guidelines For Selecting and Preserving Items in a Time Capsule” and “Creating Your Own Time Capsule”