Snails and slugs can traverse vertical and inverted terrain because they crawl on slimy gel that is a complex fluid with non-Newtonian properties. In the most basic terms, the slime acts like a solid glue at rest, but liquifies when an adequate stress is a applied (a stress exceeding the apparent yield stress). When the stress is removed, the slime quickly re-solidifies. By exploiting this yield-heal property, a snail can keep part of its foot stuck to the wall, while other parts move forward. The picture shows the bottom of a Leopard Slug, Limax maximus, during locomotion.
Here is a video of the experimental setup ((987 KB) (Randy Ewoldt).You can see the waves (1.1 MB) in the mucus while the snail moves (Brian Chan).
The mucus is studied (680 KB) by particle image velocimetry (PIV) (Randy Ewoldt).
You may also look at Randy Ewoldt's personnal webpage.