1. Show radically different ideas, not variations on a theme. If you don't show dramatic difference between comps, clients are much more comfortable playing art director. If you have one great layout and still need to generate multiple concepts, don't just iterate on the original.
2. If it's interactive work, clearly explain your documentation. I don't like to show "concepts" for UX, but when shifting from wireframe into multiple screen design concepts, be prepared for the client to react to details that were clear and "approved" in the documentation. Clients can start mingling elements of multiple screen designs to "solve" the problem, while it's often best to return to the wireframe and update the information architecture of the page first.
3. Show fewer concepts whenever possible. Any more than three, you're at risk.
4. Get them to love your idea before they love your execution. Be prepared to design less, not more. Microdetailing your comps can lead to your filigree being folded wholesale into the final design. This is why I always recommend showing sketches whenever possible, and getting the client to swoon over the direction before the visual.
5. Make sure the content structures don't wildly differ. If you introduce new types of content to a page -- i.e. "This idea has a pull quote, while in the other one we tried long copy..." -- you're introducing different layout features that could be mixed and matched. For interactive work, this isn't as much of a concern, since you'll be working off a wireframe. Though this could be a problem. (See #2.)
6. Articulate your creative strategy for each comp before showing the work. I find this the most important rule of the bunch. By drawing a boundary around what you're trying to accomplish, the details feel integrated with the concepts and don't "peel away" so easily.
Have any ideas to add?