Re: I want to sabatoge a relationship..
There are many reasons for an evolved, potent human to wish to sabotage a relationship.
Of course, the OP of this thread seems to wish to do so as a result of fairly normal, low-scale desires. And the implicit moralities of most replies to her have dealt in the same little-person currency.
But, consider that interpersonal ties are essential to the maintenance of societal structures, and that the ability to deliberately and precisely break or alter these ties confers, necessarily, a greater ability to deliberately manipulate societal structures generally. Thus, anybody interested in altering the way that humans around them operate in accordance with a vision, or anybody who simply likes playing with and rearranging the building blocks of the human institutions their lives are immersed in or subject to, or anybody who is interested in causing deliberate social change in service of whatever motive they may happen to have (and these could be infinite, and beyond our conception) will, if they have the potential to be effective, find value in this topic.
Further, the emotional intensity involved with this kind of situation, and its pertinence to the identities and self-understanding of participants, is a useful tool in effecting deliberate changes to individuals. The potential/current partner need not be disliked by the person doing the sabotaging, but their emotional significance to the individual of import is valuable. They can be drawn into a Dionysian, living drama which tests, alters and thus, potentially, strengthens the individual concerned. One gains, one loses. This is life, and its danger, made conscious for the purpose of elevating an individual beyond his current limits. In short: sabotaging a relationship can form part of a very potent initiatory method, ultimately aimed towards giving the individual involved the opportunity to live more intensely and numinously and, perhaps, attain wisdom and strength through adversity.
In other words: anybody important (which I mean without any shade of moral evaluation, and in terms of sheer potency to be important to other people en mass) will find this topic important. Those raging against the OP are essentially modern serfs, and are likely devoid of the artistic vision necessary to conceive of the many forms of living art which sabotaging the bond between two people might be a necessary part of.
For those who like to play, though, and who need no little-justification for such, I will relate my practically oriented thoughts on this matter. The most likely problem, for most reading this, is that you are little. A person of psychic magnitude, who understands how to create worlds within worlds-- that is, who understands how to bend and morph reality around himself-- typically understands the necessary steps on a visceral level.
The next issue is that the OP is asking for a generic formula. While describing likely methods might be of some, limited relevancy, in almost all cases it will be much more effective to have an empathetic understanding of the unwilling characters in your contrived, living drama. This empathy will reveal to you what aspects of their psyche can be deliberately used as leverage, and how this would best be done.
Thus, it is almost impossible to provide further high-quality advice until more details about this specific situation, and its participants, are known. And even then, the very asking indicates a high likelihood that natural ability and direct, immovable understanding of what is necessary is absent. If this is the case, it cannot be remedied via the medium we are using (a forum topic), and possibly not by any other medium either. The inherent potency, and perceptiveness, necessary is simply absent.
To those who do have this potency, the most useful advice, I think, is to experience and experiment for yourself, and thereby understand on a practical and experiential level. But... most to whom that advice applies don't need to be given it.
(Tag for google: Order of Nine Angles)
probably for the best 
and that's why you had to seek advice to end it.
