We all know the problem. While your trophy list grows and as more games you install, the synchronize process of the trophies becomes slower and slower. Now I found a simple solution to fix this problem and speed it up.
Here the How-To:
1. Shutdown PS3
2. Hold the power-button till the PS3 shuts down by itself after some beeps
3. Hold the power-button down again, till you hear a "double beep"
4. Unhand the button; now the service menu starts
5. Choose option 4 "Rebuild Database"
6. Done, now your PS3 updates the trophies very fast again