Start the call by telling the rep you were thinking of canceling you contract as you got a better offer from ____(Bell, telus, wind etc.)
They will ask what kind of offer- here you tell them what you want and for how much.
The typical rep won't be able to do any thing, so they will transfer you to the cancelation/retentions department so they can try and get something that works for you, and so they can keep you as a customer.
I went through this the other day (with Fido), I told them I want to upgrade to the iPhone 4 when it comes out so they just gave me the offer and I just call back the day of/day before of the when I go get the iPhone and they will switch my plan over.
I ended up getting:
200 minutes
Evenings and weekends starting at 5pm
2500 text messages
Caller ID
Voicemail
6gb of data (don't think you can get this any more as I had this from the iPhone 3G launch)
All for 55 dollars (or was it 50? dont remember).. No system access fee or 911 fees.
For what you want, you should be able to get it for around 30-35 dollars. I remember there being a retentions plan with 200min+5pm evenings and weekends+1gb data for really cheap.
Just make sure you come off like you are willing to cancel. All the major carriers will be gettings the iPhone 4 and they know this.
IF after your call you can't get something you are happy with just tell them you will think about it. Then call back and start over.
In my situation I'm only into my 3GS contract one year....I pay around $98 a month for my plan.
I want a much cheaper plan but worry that if I call retentions before the iPhone comes out they will deem me unable to early upgrade based on the fact my plan is no longer in the $100 range.
This is what I have and pay for currently....
$30 - 150 day minutes plus double your minutes and an extra 50 mins for a total of 350 day time
$9 - for early evening calling starting at 5 P.M.
$2 - Name display
$20 - iPhone value pack (call display, who called, visual voice mail, unlimited text & picture messaging)
$30 - data package 6GB
Plus I get a $5 credit every month.....