The performance of most detectors can be improved dramatically by subtracting a dark frame; that is, an image taken under the same conditions but with the shutter closed. This removes dark current, which is sensitive to both temperature and exposure time.
Unfortunately most DSLR cameras cannot take an exposure with the shutter closed, and there is no temperature control or cooling. This makes it difficult to reuse dark frames since they cannot be acquired at the same temperature, unless you acquire them during your imaging session.
Most newer DSLR cameras have a mode that takes and subtracts internal dark frames. By activating this mode, via the camera's built-in menus, you can have the darks taken immediately after each exposure and subtracted in the camera before download. While the exposure time doubles, the convenience and accuracy can be worthwhile. Unfortunately this means that half of your "dark time" is spent taking dark frames, which is not a good use of the limited time available.
If your camera does not have internal dark frame subtraction, then it is strongly recommended to use the RAW image download option. Dark subtraction does not work as well when the images have already been converted to color. A typical approach is to pause in the middle of an exposure sequence to shoot darks, or to shoot darks at the beginning and end. Another common approach is to monitor the ambient temperature at the telescope, and use dark frames that were taken at the same temperature (or as close as is practical).
In MaxIm DL it is possible to configure a filter wheel to act as a shutter. You set up one "filter" with some kind of opaque material. Then you can tell MaxIm DL to use this slot for dark frame exposures. The Use Filter Wheel As Shutter option is available on the Setup Tab under Options.