jkerekes's "use a dongle" must be option 0 then....
actually it is the easiest way, I use my dongle to activate software on my home desktop, two laptops, and a couple of clients' machines when working on-site.
No worries about not being able to connect to an "activation server" when needed.
Plus it is a 2GB memory stick, so the .LIC license files can be transported with the activations.
The only drawbacks are....
1. You must remember to take it with you wherever you go... (not failed yet).
2. You have no "plan B" if it stops working.
3. It sticks out the side of my laptops, not the rear, so is a bit exposed to being knocked.