Learn how the phone works. This is no iPhone and the settings with this phone are plentyfull. Have in mind that after you flash a Cyanogen ROM for instance, you will have 4 times more settings to play with, hence the advice to learn the basic functions on your N1. But anyways, you will come to a certain point where the GEEK in you wakes up and you wanting MORE..
DO also know that when you want to root your phone, you need to unlock you phone. Doing this will VOID YOUR WARRANTY. I will also advise you to use your phone for a period of time to see that there are no issues with hardware and build of the phone (buttons, dust under screen, trackball not working, not chargin, loose connectors etc etc..), just to be safe. Coz once you unlock it, there is NO going back to lock it again (at this point)
Setting up ADB and Recovery can be the most struggling with drivers and interface but once that is passed and you start using recovery on a daily or weekly basis, is all smooth ride from there. I guarantee you many hours customizing and experimenting with this phone than any other non-andoid phones you have had in the past.. is pretty much an addiction. i have rebooted this phone more the last 3 months than any other phones in my whole lifetime
Rule of thumb is always have a recent Nandroid (backup done in recovery) before flashing. Flashing ROM's are usually safe as safe can be. Kernels are not risky to flash either, but remember that Kernels are the ones that controls voltages and speed of you Processor and other things, and flashing wrong Kernels can result in non functioning behavious in your OS and worst case, the processor overheating, so know what you are doing when flashing Kernels. At one point you will start trying out kernels because if you are on a hunt for more battery life, the ROM + Kernel combination are the main things that makes this happens.
Then , a last note: I addvise you to NOT update radios if you don't have any problems with the one you have. The Radio's are the one that controls your ability to call controls your phone's antenna and Wi-Fi. If you DO flash radios and happen to terminate the process, i guarantee you a bricked phone. So beware. If you upgrade to FROYO, one of the processes is flashing a new radio. So let things runs it course once in process.
Have fun trying to optimize your phone to last as long as it can on one battery charge...