Apologies are best received when they are spoken sincerely. The only thing worse than an apology that is rude, sarcastic, or acerbic is an apology that includes a single three-letter word.
I'm sorry, but it wasn't my fault...
I'm sorry, but if you had only...
I'm sorry, but I wasn't the one who...
An apology without ownership of a wrong, is not an apology at all. I don't know for certain that this is in fact a quote from Mr. Benjamin Franklin, but it's a good one nonetheless:
"Never ruin an apology with an excuse."
If you mean it, say it with meaning. And never make an excuse for it. Own it. Chances are, the person on the receiving end of the apology will accept it far more quickly if you take responsibility for your words or actions (or in some cases the lack thereof) and nix any hint of a giant "but". It will only make things worse if you try to rationalize what you did it after saying you are sorry.