FemaleScienceProfessor has a post on pre-accepted students writing professors for advice on grad school, queries regarding availability, etc.
In the end, I don't think that e-mailing professors in such a manner actually helps your application. What does help is *real* getting your foot in the door, such as doing REU's or other summer programs available, or getting a job as a research programmer (this is good if you're wanting to take a year "off" between degrees anyway). Asking professors about this sort of availability is a good idea-- the programs aren't always easy to find on department main pages. Of course, you have to do that a year before your apps are due.
Also, if you have the opportunity to go to a conference while you're an undergrad is helpful-- you get more of a chance to demonstrate you know what you're talking about. And if nothing else you can at least talk to other grad students, who are less intimidating and more likely to ask you to join them for a beer.