1. Get yourself a low cost netbook.
2. Install not bloated linux (like archlinux) or FreeBSD or OpenBSD (in order of growing paranoia).
- make sure that the above is done with ecrypted partitions and swap (plenty of guides on the net).
- make sure that the above is done while offline as much as possible (for truly paranoid ones).
- make sure that you do not not even configure wireless hardware, let alone using it
- physically plug in Ethernet cable when you need connectivity for a minute or so
3. Install bitcoin client, generate a bunch of bitcoin addresses (current account)
4. Over time, transfer in small amounts (i.e not all of it in one go) your funds from your existing client to the addresses created in step 3
5. Keep this used exclusively as bitcoin client and nothing else, plug in Ethernet cable when you need to transfer money.
6. Keep this hardware wallet safe.
7. Creating a bitcoin savings account and making secure backups is still need to be done as described in multiply guides elsewhere.
P.S. Do not forget your passwords.
EDIT: your mileage may vary
BombaUcigasa made a very reasonable suggestion:
2. Install not bloated linux (like archlinux) or FreeBSD or OpenBSD (in order of growing paranoia).
Preferably change this rule to:
2. Install pre-2009 not bloated linux (like archlinux) or FreeBSD or OpenBSD (in order of growing paranoia) - don't install updates.This to make sure that you use code generated before bitcoin existed, and as such there is no local vulnerability and never will be since you don't update it. (remote access vulnerabilities still present).
Considering that at most, as far as remote accessible services are concerned, you are only occasionally running sshd and I do not recall it having any fatal remotely exploitable bugs after 2008, this will do. You would have to compile bitcoin yourself, and this may cause problems due to out of date libs, though. I did not try it, yet.