Here is a few tips:
1. When you are wanting to create some effects in your signature ("effect c4ds") you can just set the
layer to lighten or screen to get rid of any of the dark spots a keep the highlights.
2. Another option is to go in to the "channels" tab and select the channel that has the best black/white ratio.
Then, while holding CTRL, left click on the selected channel and you should see a selection.
Then go back to the top channel and go into your layers pallet and copy the selection.
You can do this with any of the channels depending on your needs and plans on the usage of the c4d.
3. If your just want to get rid of parts of the c4d, then you'll want to lower the opacity of the brush.
This way, you slowely erase portions of it and do a neater job.
Or you can use a mask and just make portions of it disappear. This way you can fix it later if see things you want to change