If you want to make your sims invisible "forever".
Do the following:
Buy a teleporting machine
Select the sim you want to make invisible.
Let him/her teleport
In the middle of teleporting quickly press buy mode
Delete the teleporter and thats it!
Note: you can make all of your Sims invisible but invisible person can't talk to another invisible person and also don't let them teleport again or they will return back to normal.