I've been through two and a half layoffs.
1st was managed like crap in November of 2001 working for a financial software company. Over the course of 3 weeks they laid off a bunch of people each Friday totaling 1/3 - 1/2 of the company. Each time saying that they done with layoffs for the foreseeable future. Each cut was made primarily by start date leaving their new application without enough workers and some parts of it's development without any workers.
2nd was done ok'ish in March of 2008. Again in finance, this time making customizations to the application I helped write back in 2001. We knew there was some trouble, it had been well communicated. We just didn't know how close that trouble was. A deal to get buyers for our investments fell through and the company hit their break point for funds. An all hands meeting was held stating that the company was going to close. Contractors would be let go immediately unless they were working on something absolutely necessary. In a week announcements would be made on a layoff schedule and they reminded us of and explained the details of the company severance package. During that week work everyone continued on necessary work and documented what was needed to be done to mothball everything. Management met together and put in their requests for who to stay and how long. Emails were sent out stating that some would be asked to leave immediately, others stay for some time longer, some until the end of June, and a few until our portfolio was emptied. Then they had the meetings with each person, the division head, and one member of HR. If you went with what they wanted you would be given an extra month's pay. You could change it, but then you'd lose your severance and the month's pay.
3rd was more a half lay off. I was working under a research grant and well before our grant was up for renewal it was announced that we wouldn't be getting the grant again as it was switching from research to implementation. Everyone was basically told to take their time and look for a new job or hang around until the end + 6 months. They also said they may get a new grant immediately or it may take awhile, but after the 6 months only a couple specific employes (necessary to continue grant applications) would be allowed to stay.