This is something that has needed to be addressed for some time, as the voters in the Speaker's constituency - Buckingham (at the moment) are effectively disenfranchised.
There are a few ideas in here...
http://www.democraticaudit.com/?p=1288
This seems of the best..
"One option may be to transform the office of Speaker, so it is no longer carried out alongside the duties of a regular MP. In this scenario, Speakers would resign their parliamentary seat once selected for the post, but remain in Parliament with a special status. They could perhaps be given a token position such as MP for the Palace of Westminster, if the weight of parliamentary tradition proves too hard to cast off. The constituency would then elect a regular MP to replace the new Speaker."