"Yea I will definitely be there"
"We will hang out this weekend for sure"
"I will call you"
I would imagine most of us have said these things or variations of and not followed through. This is not acceptable. Some of us, myself included are guilty of this on occasion. For others it is a habit. If it is done often enough, you can't take their word for a damn. People commit to things they have no intention of doing. They think its ok because they do not want to hurt peoples feelings by saying no or turning them down. They fail to realize that the impact of agreeing to doing something and not actually doing the act hurts more than initially saying No. It is ok if you are unsure or wavering if a guy or girl wants to go out, or if your sorta friend is having a bday party. This is what you can say.
"I'm supposed to do X, but well see"
"I'll try to stop by"
"I'll let u know"
These things don't even have to be true but they at least don't let people get their hopes up or have them pencil you in only to have you renege in the 11th hour.
If you find yourself committed to something that you regret, you should still go anyways to keep your word. The only excuse you really have is an emergency, but make sure to not use the same excuse on the same friend twice.