Gizmodo has a detailed walkthrough that guides you through the process of getting homebrew apps running on the PSP Slim (or any PSP, really). They skip the whole step of enabling service mode on your battery, since there are some cheap service mode batteries available for $25 that will save you the effort.
When you’ve completed the walkthrough, you’ll have the latest hacked firmware installed which will allow you to run homebrew apps designed for the PSP, as well as any old PS1 games that you own and care to rip.
This is a nice HOWTO and very easy to build from salvaged material. What I encountered however was that volt output from serialport varies both in pinouts and volts from PC to PC. I still havent been able to make this work in practice but it works great hooked to powersupply and LED to show activity.
If anyone knows of a solution, drop me a line