If you are talking a HUGE cash like the Sunday Million, I'd have to pretty much agree with Ref (shocker

)... on a $200K win I would keep a good $15,000 in my account and start playing $50 - $200 MTTs.
With a $50K win I would have to cash out $45K and would still probably start in on the $50 - $200 MTTs, upon losing a couple thousand I'd have no problem moving back to my normal level.
It is certainly tempting to throw a good portion back into the mix hoping to hit it big... when I first started playing the game I hit an MTT for $4500 or so of which I immediately cashed out a good percentage but the higher stakes bug swept me off my feet (my graph on SS is funny, I go STRAIGHT DOWN from $0 to
-$1300 LOL).