My cover was always, "lightly-armored fighter" and I depended on the lack of curiousity of others to keep the illusion alive.
That has the advantage of no one asking you to use your thief skills, which is good, because the thief skills suck for thieves, and suck worse for assassins.
Sorry, I can't find or remove traps. Oh well.
It gets a little dicier once you learn to make poisons (which I knew about, because I also DM'd, so I read the section in the DM that said I could do it, and to never tell players that they could).