It is a dangerous area; especially because it creates a chance for the partner to ''destroy'' the place. Do not do it at your own home, he/she can make a mess out of it (like smash your favorite objects).
2. At the office
If you work at the same office, avoid making a scene in front of colleagues, especially because they may gossip. There can be public scenes, tears and cries and you do not want people to see this.
3. On vacation
Do you think that if you plan a vacation this will make a break-up easier? If you want to take him/her to a nice place, save the bad news till you get back. Let him/her relax and enjoy the vacation - maybe it will make announcing the bad news easier to accept.
4. In a bar or restaurant
Do not think you will surprise him/her offering food and drinks to make things go smoothly. Alcohol can make him/her aggressive or talkative. You do not want anyone to know your most intimate characterization like ''he/she was a lousy lover'' or ''he/she is a cheater''.
5. On the Internet
Suck break-ups are easier for the one who has the initiative but think about your partner. Your lover deserves better, so you need to explain the reason face to face.
It's not easy to end-up a relationship and you must think about everything. Think your partner will ask questions so you need to be prepared. Explain what went wrong but say you will always be there for him/her.