Like the other suggestions state take it nice and easy especially on those machines...theyre a bit of a handful.
A recommendation I have is get a decent set of tools.
Some are hard to grip (holding and grip into the screw slot itself).
Happened to me alot with the classics and a few unibodies doing work on.
I just picked up this toolkit:
http://eshop.macsales.com/item/Newer Technology/TOOLKIT11/
Has a nicer grip holding and fitting inside the screws; no wear or anything.
Definitely improved time replacing drives and memory