save PSCredential in the file

Building on Briantist & Graham: This will ask for a credential and store it on first run, then reuse it on subsequent runs from the same code. In my case drive H is the user's home directory, used for tidiness, not security.

# the path to stored credential
$credPath = "H:\Secrets\Cred_${env:USERNAME}_${env:COMPUTERNAME}.xml"
# check for stored credential
if ( Test-Path $credPath ) {
    #crendetial is stored, load it 
    $cred = Import-CliXml -Path $credPath
} else {
    # no stored credential: create store, get credential and save it
    $parent = split-path $credpath -parent
    if ( -not ( test-Path $parent ) ) {
        New-Item -ItemType Directory -Force -Path $parent
    }
    $cred = get-credential
    $cred | Export-CliXml -Path $credPath
}

This block of code can be chucked in any script that needs it and the problem is more-or-less solved from then on.

Could possibly also check for a successful credential before writing it if the application permits it. Note that if the password changes the user must delete the file. [edit: missing parenthesis in code]

Update on non-Windows Platforms

A lot has changed since this answer was first written. Modern versions of PowerShell are based on .net core, and run cross-platform. The underlying type that enables this whole answer is called [securestring] and the security and encryption that backs it comes from the Data Protection API (DPAPI) on Windows, which is not open source and not available cross-platform.

As such, while you can possibly use the same code here on non-Windows platforms, do note that there will be absolutely no encryption backing it.

tl;dr: don't use this on non-Windows platforms!

More information available in this excellent answer on a related question.

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To store and retrieve encrypted credentials easily, use PowerShell's built-in XML serialization (Clixml):

$credential = Get-Credential

$credential | Export-CliXml -Path 'C:\My\Path\cred.xml'

To re-import:

$credential = Import-CliXml -Path 'C:\My\Path\cred.xml'

The important thing to remember is that by default this uses the Windows data protection API, and the key used to encrypt the password is specific to both the user and the machine that the code is running under.

As a result, the encrypted credential cannot be imported by a different user nor the same user on a different computer.

By encrypting several versions of the same credential with different running users and on different computers, you can have the same secret available to multiple users.

By putting the user and computer name in the file name, you can store all of the encrypted secrets in a way that allows for the same code to use them without hard coding anything:

Encrypter

# run as each user, and on each computer

$credential = Get-Credential

$credential | Export-CliXml -Path "C:\My\Secrets\myCred_${env:USERNAME}_${env:COMPUTERNAME}.xml"

The code that uses the stored credentials:

$credential = Import-CliXml -Path "C:\My\Secrets\myCred_${env:USERNAME}_${env:COMPUTERNAME}.xml"

The correct version of the file for the running user will be loaded automatically (or it will fail because the file doesn't exist).