There's two issues here:
1) Getting out of the relationship.
2) How you get out of the relationship.
It's important to treat the other person with a little respect when you're getting out of the relationship. There's good ways and there's kind of crummy ways. You'd prefer to use one of the good ways.
However, that doesn't change the fact that #1 is more important than #2, even if you're only capable of one of the crummy ways. Don't confuse your priorities. If you stay with the person because you're not capable of one of the good ways of dumping them, then what you're doing to both you and them is worse than if you resorted to one of the crummy ways.
So, in rank order:
1) Tell them in person why you're no longer interested in them. Be honest and straight forward, hopefully without being malicious and mean. But make it clear that the only option is ending the relationship. This is not the time for a "criticism sandwich" (telling them their good points, what they need to improve on, but that they're a good person overall). That won't end the relationship - instead they'll hear they need to do such and such to keep the relationship alive.
4) Tell them in a letter why you're no longer interested. Same guidelines as in person, but you don't have to face them saying they'll change and the relationship will get better, tears, etc. You should tell them in person, but if a letter is all you're capable of, then it's all you're capable of.
7) Tell them on the phone. This is actually worse than a letter because the phone will give you both poor communication and her arguing the relationship doesn't have to be over.
9) (or maybe #3) Send them an e-mail. I guess in today's technology, this might actually be as good as a letter. For traditionalists, this seems a little tacky. But you do what you're capable of doing and who knows? Maybe your handwriting is so bad that you'd be better off sending an e-mail. Besides, for younger people, I think e-mail has completely replaced the concept of letter writing, anyway.
17) Send them a text message. Short sweet. Hard for her to argue. Especially since you can decide to quit responding to her text messages. This is a pretty tacky option, even for younger people.
32) Just quit contacting them period. Don't call, text, or e-mail them. Don't answer or respond to any of their calls, texts, or e-mails. Eventually they'll get the picture that you're no longer interested. This is worse than just tacky. It's a cowardly method and a cruel method. None the less, it's better than option 7.
47) Sleep with her sister, or worse yet, her mother. She'll dump you and your problems are solved. Well, at least your problems with her. If her mother's a cougar, then there's a possibility of opening a whole new can of whirled peas. As bad as option 7 is, it's better than option 8.
51) Run over her with your car. This has some really bad consequences if anyone can prove you didn't do this accidentally. Definitely don't do this! None the less, this is still a better option than option 9.
52) Stay in the relationship. This is bad, bad, bad. Best case is that she eventually catches on to the non-verbal clues and dumps you. Worst case is that she doesn't catch on and the two of you eventually get married, have half a dozen kids, and live the rest of your lives together with you hating her guts while she thinks you love her.
I don't care how tacky the method you use to dump her, but you dump her.
But you really should work on developing some of the better methods for dumping someone.
And, obviously, I left out a few of the 50 ways you could leave her.