marking of trademarks: (R) vs TM

I was asked again today about the difference between marking your trademark with an ® ("R" in a circle) and with ™ (small "TM").  There seems to be a lot of confusion.  The answer is straightforward - you can only use the ® if your trademark has been registered.  You can (and probably should) use the ™ any time that you want to put the world on notice that you are using a particular word, slogan or image as a mark in trade.  

Just to clarify - there are a ton of rules re: what can and cannot be a valid trademark. Simply putting the ™ beside your mark doesn't provide the protection that registration does, and also doesn't guarantee that the mark will be enforceable, but it does provide evidence of use in the normal course of trade (which is one of the requirements for enforceability under the Trade-marks Act).