You only ever need to back-up your wallet once. The easiest way is to use "dumpprivkey" and print out the privkeys for the accounts in your wallet.
No matter whatever happens you can download and install a new client and them import your privkeys and have your money back (unless someone hacked your wallet and took your coins).
If you're not going to be using your wallet for a while, dump the privkeys and remove the client from your computer entirely, then import the privkey on a new client when you're ready to spend.
Only once? It is more than that if you got new addresses in your wallet after that first backup....No matter whatever happens you can download and install a new client and them import your privkeys and have your money back (unless someone hacked your wallet and took your coins).
If you're not going to be using your wallet for a while, dump the privkeys and remove the client from your computer entirely, then import the privkey on a new client when you're ready to spend.
I had not considered the thought of exhausting the initial keypool, I apologize (I haven't come anywhere near exhausting mine yet so didn't occur to me)...
But he is correct, If you create more than 100 new addresses, you will want to back-up your wallet.dat again and it is always good practice to back-up your wallet any time before upgrading the client just in case you've made any changes to encryption or exhausted the keypool, etc... that you might have forgotten about since your last back-up...
