Cut out all the UNnecessary add ons suck as "call waiting, call forwarding and caller ID....if the phone rings and its someone you'd rather not talk too, you can always say let me call you back....better yet, let an answer machine take hte calls.

When all I need is a Yes/No answer, I often make an SMS instead of a phone call, and to avoid the other one to have to pay, I add these 2 lines:
"1 missed call = yes
2 missed calls = no"

and usually they do so

Also, if you're a long typer on SMS, try to focus on the character count (usually above right of the screen), normally it's 160 characters per SMS. For each cut of 160 characters, you pay for 1 SMS. So if I made for example 163 characters, I'll pay 2 SMS at once.

Sometimes the counters are written like that: characters left/pages.
So 145/1 would mean you have 145 characters left to type and it's in 1 SMS. 92/2 would mean you're already at the 2nd SMS with 92 character to type in it. etc.

Sometimes, the counter it written like this: max. characters left/pages.
My Nokia 5000 writes this: 1000/1, but once I've reached below 840/1 (i.e., 160 characters later), it changes to 839/2.

In those limit-cases I choose wether I add information to clear myself and full up the next page or if I make some words shorter to have one page less.

It sounds a bit silly but I've noticed some price cuts around 5 to 7 EUR of difference per month when watching out of that. That's because I'm at one carrier and most of my friends are at the other, and Belgian carrier prices are very high once it comes to contact to another carrier.