iPod Delete music/movies after syncing...?

I'm sure this has been asked a million times, but I can't seem to find it right now...

What's the best way to transfer music/movies to my iPod Classic, then delete the files on my computer/iTunes WITHOUT it being deleted from my iPod the next time I sync? The best solution I've found so far is to use "manually manage iPod" BUT it seems like a pretty big hassle to transfer songs to iTunes, then to iPod, then delete from iTunes...is there a simpler way to just transfer files to my iPod and delete the old files?

Thanks!

Thanks for your reply!

Just to clarify, what you've described is the easiest way...but if I've already deleted the reference to the files from iTunes, that's fine too (i.e. they won't suddenly disappear from the iPod)? So far they haven't disappeared when I deleted both the original files and the reference to the files from iTunes but I thought I should ask just to be safe...

Also, I agree it's a good idea to have a backup...but this is actually my mom's iPod and her computer's HD is about 1/3 the size of the iPod's HD...so it's impossible to back everything up (without buying another HD).

Hmm...so your method only works for manual mode as well, right?

Thanks again for your help.