When to Sell a Dividend Paying Investment

I hate to sell stocks that I own. Why? One of two reasons:

1. I probably screwed up in my analysis of the company and made a dumb decision to buy in the first place;
or
2. Something has happened to the company that is totally beyond my control and I am powerless (e.g. the company has to pull a major product from the shelves because it is linked to deaths - a la Merck and Vioxx).

My selling principle tends to lean towards the "Buy and Hold" mentality. I believe in the likes of Warren Buffet who has said, "Our favorite holding period is forever", or "I buy on the assumption that they could close the stock market the next day and not reopen it for five years." I do however strongly believe that this does not mean that an investor should not have a selling strategy. There will be times when it may make sense to unload a stock.

Here are the "rules" I TRY to live by when deciding when to sell - sometimes it just doesn't work though:
1. When there is something that I absolutely must buy because I am so confident that the company will provide me with much better returns (dividend yield and price appreciation) and I need the cash to do so, I sell something.
2. I sell my losers, not my winners. Winners tend to keep on winning and losers tend to keep on losing.
3. Keep emotion out of it - I make decisions based on the facts and not the feelings.

That's it. Pretty basic. Any other rules that you follow when deciding to sell a stock? Let me know.