Starting with a sub

Hiya!
I have been recording/mixing for years now, and have never used a sub.  Ever.
Nobody has complained about my low-end, but I would like to pursue the purchase and set-up of a sub.
 
Where do I start?  Measure the room, search various forums for potential models that might be good to look into, etc?
 
The whole sub thing is a mystery to me because I've never thought about having one, and I'm actually afraid to make my mixing worse if I purchase the wrong one or set it up poorly!

you've already become accustomed to your monitor and how they translate on other systems. thats great. adding a sub will no doubt change that and you'll have to learn that all over again. not to say that it a bad thing. I think you should get one if you feel the need or just to test on Bass heavy productions. you can always disengage the sub.
What monitors are you using now?
what low freq's do you think your missing that your monitors are not producing?
as youve been mixing a long time Im sure you know all these things.
im just conversing really

Hey guys!  Thanks for the replies, wasn't sure how often people were on this forum as I never post here :)
 
My monitoring is through Yorkville YSM1p's.  Pretty affordable monitors you can get up here in Canada (eh) at Long and McQuade (our version of Guitar Center).  I've never been able to bring myself to upgrade to a great pair of monitors since I've always been able to turn out good mixes on these.
 
For room treatment, I have about 20 pieces of Gik Acoustics panels all over my room and 4 corner-traps.  The traps do NOT go to the ceiling, however.  They go about 1/2 way up.  I have a 7 foot-tall room with carpet.  I mix/track in my basement.  It's an L-shape and I mix in the shorter area so the walls are more equilateral (if that's the right term).  To sum up, my room isn't grand but it gets the job done :)  I feel pretty lucky to have a room I can work in.

My low-end isn't too bad.  I honestly would not feel comfortable mixing, say, a NIN record or certain types of metal/dance.  I have mixed dance music and actually won awards for the client... so my low-end isn't horrid I guess haha.
 
But I do know that when I'm mixing metal and have a sub-drop or two, I have to take it to the car to reference.  I just don't feel the bass.  Or hear much past 50hz.  I really rely on reference tracks and analyzers when dealing with stuff past 50hz.  I can hear a bit, but can't feel enough to be super comfy.

Thank you!  The advice on placement was fantastic.