I was able to find other stocks I though had similar or better prospects than my company stock. I tracked them during 2001-2003. When any of them dropped to a new low new stock/old stock price ratio for the past 2 years, I sold my company stock and bought enough of the new stock to make up about 2.5% of my portfolio. If the ratio fell another 15%, I bought the same number of shares again. I felt I was getting a good short-term deal, and wasn't hurting my long-term prospects either.
I'm about 40% ahead of where I'd be without diversification, including tax costs. And my cost basis is now much higher. I no longer have any of the original company stock.
As far as taxes go, watch out for the deduction and exemption exclusions as your AGI goes up. Your effective capital gains rate can be more like 25% than 15%. However, if your AGI is high enough (including selling lots of company stock), eventually the exclusions max out and the capital gains rate is back at 15%. You might want to try to sell lots of stock in one year if you can hit that 15% rate. Otherwise, you just have to hold your nose and pay the taxes. Your cost basis goes up, so if you look at after-tax value, it will seem a little better. The 15% rate is scheduled to disappear in a few years, so convince yourself you need to take advantage of it now.
Good luck!