My method is to buy in for the maximum amount at a table, and quit when I am ahead.  How much ahead?  Anywhere from 10% to 100%.  My winnings are seldom at risk, because I take them off the table when I leave.  When I sit down to play again (hours later, or the next day) I once again buy in for the maximum amount at the table.  This method works (usually) because I have a slight advantage against many of the players I play against.  When I know (from experience) that someone is a much better player than I am, I avoid challenging them.  I try to win from someone else.  There is always someone dumber than I am, or drunk, or stoned, or more easily intimidated, or more impatient.