So the best thing to do to improve contrast is move you and the telescope away from contrast-robbing streetlights. However, if you are like me, you may not be able to drive outside the city every time the sky is clear. That takes effort and planning and time and gas money. All of which are typically in short supply. Lets continue to examine the causes of reduced contrast.
Streetlights are a big one, of course, and weather plays an important role as well.
But there really isn't much you can do about the weather except wait it out. Light scattering within the telescope itself is, however, an area that we can improve. In the city, there is lots of reflected light around, again mostly from streetlights. The light reflects off trees, the neighbor's house, parked cars, the ground, everything. All of this light is bouncing around at all angles and some of it will be angled into the telescope.