Laptops are causing back, joint and neck strain. Elevate the computer and don’t rest it on your legs, resting it on your legs causes you to look down at the computer which puts a strain on your neck. Also, don’t rest your wrists on the edge of the laptop, this causes nerve injury:

Nicola Hunter, a physiotherapist and occupational health specialist, said that hand and arm pain similar to repetitive strain injury was easily induced by resting wrists against the edge of a laptop.

She added: “There’s evidence that it stops the nerves and tendons moving as they normally would, and this can cause nerve injury.”

(via)
This is the problem that I’m having now. I’ve been getting a pain in my right wrist, a very sharp pain and I’m unable to pick anything up because of it. It went away during my vacation but it’s back now. I guess I’ll have to start using a table but if I do that, doesn’t that defeat the purpose of having a laptop? Why not just use a desk top?