You can't turn up the dialog specifically, but considering that the majority of the dialog comes from the center channel, you can bump up the center channel volume.
The correct way to do things, however, is to calibrate the entire system so that the channel levels are balanced. Then you can adjust the center channel up or down on the fly if you feel the need. When you shut the receiver off, it will revert to your saved settings.
Note that many receivers include dynamic compression, sometimes called 'midnight mode'. It only works on DD tracks but it reduces the peaks, raising the average level, and thus sometimes helping to make dialog more intelligible.
Additionally, speaker placement and room treatments can help alot with dialog intelligibility.