• Complete sale: Incur the tax and diversify to reduce risk.  This is the quickest method to diversify.  However, if the assets are not in an IRA it also requires recognition of gain (you will have to pay the tax man).
• Give some of the shares in kind to a charity:  If you are already giving money to a charity, you can gift them the shares of the stock.  Since the charity is a non-profit organization, they can sell the shares and avoid the tax.
• Create a Charitable Remainder Trust (CRUT).  With a CRUT, you create an irrevocable trust.  The shares of the stock are used to fund the trust.  Once the shares are in the trust, the trustee can sell them.  The trust will then pay you an income for a set period of time.  The gain is not avoided since you pay tax on the distribution from the trust.  However, you can spread the tax out over a longer period of time.  At the end of the term of the trust, the charity gets the remaining balance in the trust.
• Sell portions over a set number of years to spread out the tax recognition (i.e. 3 year plan)

Note:  Be careful when selling assets if the owner is in poor health.  In many cases, if a person passes away holding a highly appreciated stock, the gain is eliminated.  This is referred to as the step-up in basis rule. 

Disclaimer: Planning related to concentrated stocks is very complicated.  The above examples are for illustrative purposes only.  They do not reflect all of the options or all of the tax implications for investors.  Please consult your financial/legal/tax professional when considering any planning strategies for concentrated stocks.