This is very simple but awkward. If you don't want to go on a date just simply cancel it being very honest and apologetic. Do not cancel a date with a text message, that will come of as being rude. You know you can't make if either because you don't want to or something came up preventing you from making it. The ideal thing is meet up with the person face to face and start by saying you are sorry you won't be able to meet up for the date and state your reasons being as honest as possible. It is important for your date to know your reason. If it is cos you ain't up to it simply tell her or him so and make her understand. Truth hurts but people appreciate it over time.
If you can't meet up as a result of distance or time factor then put a call across and let the person know what your stand is. Text message should be the last resort but I'll still prefer living a voicemail alongside the text message so the person would know I tried calling severally.
The worst thing you can do is cancel a date without communicating. It is rude, selfish and irresponsible thing to do.
I think that people sometimes mistake honesty for rudeness. I find it best to be upfront if you want to cancel a date. Tell her that you don't think the two of you are a good fit for each other, and that you would like to cancel the date.
I don't think that finding excuses for canceling it is a good idea; it only delays the problem that you will need to deal with eventually. So be honest and have a friendly discussion with him/her about it.
Going on a dates sometimes can be tricky expecially if the personality of the person you're going out with is not really something that pleases you, however I think going on a date requires consensus and basically in other not to be in a situation of having to politely reject a date the first thing in the first place will be not to accept in other not to raise the hopes of the person who is asking for the date, it's always more better if you refuse when the person is asking, however if you've accepted to go on a date and then you're having a second thought and didn't want to disappoint the person you can call the person up tell then that you really had an emergency you'll attend and that you'll like reschedule the date.
By saying this, you've totally sounded mild the person will actually seem happy that you'll reschedule the date even if you only told him/her just to take away tension and you've turned them politely even when you didn't mention the words "NO" which is definitely important.
I met someone else. I am sorry.