Boot
For x51v- Consider upgrading to Windows Mobile 6.1 as that improves your boot speed alone. Read
this thread.
For x51- Consider upgrading to WM6.1 from Lenny:
here.
*This WILL VOID your warranty and if you brick your device, I take no responsibility.
Today screen
The rule of thumb is: the less you have on the today screen, the better.
Steps: Settings-->Today-->"Items" tab-->Uncheck unwanted items-->Ok
Memmaid
One of the BEST maintenance programs out there (Do not argue with me

). If you don't want to buy it,
here's a trial.
Go to System Tweaks-->Optimize-->For Extreme Speed-->Apply
Go to Quick Cleaner-->Scan-->Clean
Go to ReclaimRAM (Use the drop down menu in upper-left hand corner)
Go to Running Programs, terminate and prevent unnecessary programs
For advanced users only!!
Go to DLL Optimizer (Must have advanced mode enabled), be careful...