Don't do it on a Friday, mid week is best. Friday is the worst, that being said get it over ASAP and give them as much warning as possible to give them time to land on their feet. I agree with Steven cut more than you have to, a second round of layoffs means you loose everybody you would want to keep. I have never seen a company survive if it has went to that well twice.

Chances are they already know it is coming, if work is slow and not coming in or the burn rate is high and you have run long on funds then they will be expecting it. Have a very clear plan to explain to those that stay, as to why the ship is righted now and how you plan to survive. Plan to loose at least a few of your best guys, if you loose one, use his/her salary to give the others a raise or you will lose them too.

Also cut the pessimist over the pragmatist or the optimist. In a layoff environment the pessimist will kill your company moral. If you got a guy that is say 15% better than another guy but he is a pessimist and the other guy is an optimist, keep the optimist. That being said, trim dead weight if they are just not skilled let them go not matter their outlook. As others have said, look to trim, trim more than you need.