To simulate dirt, I use earthtone pastels and just brush them on with a dry brush. There should be no shiney surfaces, with the possible exception of chrome. If you use dullcoat, use it before the pastells, as it tends to make them vanish.
To simulate duct tape, you can color strips of masking tape with a silver Sharpie then cut the tape to an apropriate size and apply as desired. You can also find or make some rubbish to place inside as well.
Here are a couple of examples from my builds:
Hope this helps,
David G.