I would like to present to you two examples of perfect, spot on preparation.
In the 1995 Asian Team Championships, Mark Chan helped Singapore B to a 2-2 draw against Malaysia with this stunning win over Malaysian No 1 IM Mas Hafizulhelmi.
We fast forward to Round 6 when Singapore B was pitted against Japan whose top board player was IM Domingo Ramos. Ramos,who is now a Singapore National Coach, had read the Round 3 bulletin earlier and was thinking that Mas could have improved on his setup by placing his Bishop on d3 instead and leaving his Queen on d1 instead of playing Qe2. Hence, he baited Mark into repeating the same opening and was rewarded with a fine win.
We can see how a subtle opening concept shift in the hands of an experienced master can be very telling.
IM Domingo Ramos giving a lecture at the Singapore Chess Federation
(thanks to IM Jovan Petronic for the picture).
However, the following example is the best piece of preparation I've ever seen as I was the guinea pig the day before it was unleashed.
At 1 am on 11/2/2006, I logged online to Chessbase's playchess.com server for some blitz games. Goh Wei Ming, (who had borrowed my NIC yearbook 76 and was present and wanted to show me some discoveries he unearthed in a Sicilian Najdorf 6.Bg5 game in which Gelfand had beaten Nakamura with.
So we proceeded to the "Training Room" where our analysis mainly had me trying to defend Black (to no avail) and Wei Ming eventually proving White's advantage decisively in every line. . Finally, by 2am, we agreed that White's concept is extremely dangerous and Nakamura could have improved and given Gelfand a headache with Wei Ming's idea. The next day, at the Singapore Championships 2006, Wei Ming was matched with IM Chan Peng Kong. Lo and behold, the latter played the Najdorf and Weiming struck paydirt!