How I raised my founder's disguise and stealth from 0 to max was by rescuing a sweatshop worker from the Garment Maker's in Industrial and getting them to make me a security uniform, and then waiting just outside the entrance to the garment maker's for a security guard. As soon as they got suspicious I left, and then re-entered. Eventually I had too much disguise skill to raise it in this fashion, at which point I started doing the same thing but inside the Gentleman's Club.
Stealth is pretty much the same, except instead of a security uniform, you strip naked and stand at the entrance to one of the apartment buildings. Your stealth skill will increase as long as you are in the room with a conservative and are naked. (I guess you're trying to hide yourself out of shame or something? How conservative.)
If you have a vehicle before you do this you will also end up raising your driving skill since you'll be doing quite a bit of entering and leaving, since even if you are undetected you still need to leave/re-enter after every point of disguise/stealth you gain because it doesn't seem to go any higher.
And now for my own skill-raising question: Is there an easy way to gain security skill? My infiltrator's security skill is at 7 and the only way I seem to get any skill gains for it is to sneak through the CEO residence or the Corporate headquarters and try cracking the safe. I only get the occasional success at the CEO residence and never at the Corporate headquarters, but it does raise my security skill by a miniscule amount (something like 0.07 every try). Breaking all the locks in the apartment buildings doesn't raise my skill anymore (neither does breaking any locks on doors in other buildings).