TarionzCousin
Second Most Angelic Devil Ever
Or have a surefire, drop-dead, guaranteed (and mechanically legal) way of him escaping. Give your BBEG a high-level artifact that allows him to escape.As to having one BBEG that a plot hinges on, don't do it, or if you must then don't put the BBEG in a position where the PCs can get at him......ever.
Players may feel cheated by this, but they should also be motivated to get the BBEG (and perhaps, follow your plot ideas).
My favorite escape clause is the "I escape but leave all of my gear behind" method. Either use gaseous form plus superfast fly speed, or something similar. That way the players at least get some loot and a clue or two when the BBEG slips through their fingers.