Ok, players/managers that have gone back to a previous club and been successful. I'll start with the obvious: Adam Hammill. Doesn't have to be for Barnsley. Go.
Stuart McCall has managed Bradford City four times, and each time he just gets better at getting sacked.
Ian Rush's second spell at Liverpool was pretty prolific, just not quite as much as his first. Mark Hughes returned to Man Utd from Barcelona an even better player than when he went there. Keith Hill did superbly in both his stints at Rochdale. That's just off the top of my head, there must be plenty of others.
Brian Deane did very well both times at Yoonited. Mourinho won the league in each of his Chelsea spells.
Bruce Dickinson's second spell with Iron Maiden has yielded five studio albums. The Stone Roses were arguably a much better proposition live after John Squire returned.