Well, it seems you've already identified the problem: using those chat tools is so informal and easy that it is harder NOT to use them.
I think what you did is good. You set yourself specific times when you can use them, and for the explicit purpose of talking to some friends. I think having a purpose is good. You don't want to just sign on and see what happens. You want to chat, and you know exactly how long you'll allow yourself to chat. If there is an interesting conversation going on, you can always continue it later. If it wasn't important, then it doesn't matter if it doesn't continue.
The post-it note? I like it, but you may find that you need to change it up. I have a note on top of my monitor reminding me that I want to be doing the best thing I can right now...which I only notice when it falls off. It's been there for so long that I ignore it. Changing it up with new notes every so often might be more effective at getting my attention.
I always have my away message up. If someone wants to leave me a message, they can. I can get back to them. Once I get rid of the away message, I find an hour has gone by without much productive work. So I try to make sure that if I sign on, it is for a purpose. I want to talk to this or that friend because I see that he/she is online. I'll do so for 20 minutes. Afterwards, I'll say goodbye and go about my business.
The Internet might be necessary for work, but using IM to keep up with friends isn't. You don't need to cut it out of your life completely, but you should strive to using it for a good reason and for a limited time.