Another question about Debian repos

First I wanted to say thanks everyone for all the help on various 
issues.  I'd like to specifically mention the advice on the Nvidia 
drivers.  I tried the lastest binary and it installed without a 
problem.  Also, thanks for the info on streaming servers, and those 
Icecast config files.  I'm in the process of looking all that over, 
although I've temporarily met my needs with a cheap hack: I put my music 
files on a directory of one of my public servers, and then used simple 
scripts on my other machines that mount the directory as sshfs, so I can 
listen to the music from anywhere using any music player.  (Overhead is 
a bit high -- with all that encryption/decryption going on.  Probably 
should've used FTP.)
I've been researching more into Debian's release cycle, to better 
acquaint myself with the distro.  I like how they do things, with all 
the checks and testing.  That stability and security is important to me 
(coming from some other distros whose repositories tended to be filled 
with a lot of buggy packages) even if the release cycle is a bit slow.

I'm using the Lenny sources right now, and after Lenny is officially 
released I'll probably stick with Lenny until the next stable release.  
But here's my question: What if in the future there are one or two 
packages from testing or sid that I'd like to try, but I don't want the 
rest of my system relying on those branches?  For example, say I wanted 
the lastest Amarok features, or I really needed one package that just 
wasn't available in stable.  Do I need to download and manually install 
those packages from off of the Internet, or is there a way I can install 
those from the official repos without moving my entire system out of stable?

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