Hm. Rather strange question you ask.
First of all: it depends mostly on the infrastructure you use for your bitcoin-client.
If your OperatingSystem is compromised, you probably can't guarantee security of your privateKey at all.
The Bitcoinclient supports a very strong encryption of the wallet (and ofc the private Key) since like 2 years natively.
I guess if you are paranoid and want extra security, use a VirtualMachine, install some hardened Linux-OS (DebianStable?), there make a TrueCrypt-Container with a hard password and install your Bitcoinclient there.
Ofcourse, as mentioned earlier, you can't be more secure than making an offline-wallet. Remebering your key instead of writing it down could also serve as the absolute security feature. - but i guess you don't want to go that far? :)
i saved my keys in a TrueCryp-container encrypted with sha512 and used a strong password - after this, backup copies of this container wherever you want, so you don't lose it. sha512 is not even crackable for the NSA.
2Not sure what you mean by "best". Put the private key on a piece of paper, lock it in a bank vault, surround it with armed guards and sharks with frickin' lasers? – Nate Eldredge – 2015-07-09T03:58:25.353
Hi, Thank you. Yes, cold storage is a good idea, however my mean is securing the private key during the signing process of the transaction. I need to know what is the best way to protect the private key during this process? – Mary – 2015-07-09T11:40:04.637