Just include it in your script or function:

if(-not(Get-PSSnapin | 
   Where { $_.Name -eq "Microsoft.SharePoint.PowerShell"})) { 
   Add-PSSnapin Microsoft.SharePoint.PowerShell}

By the way, I found it in PowerShell for Microsoft SharePoint 2010 Administrators by Niklas Goude and Mattias Karlsson. Powershell for Sharepoint Administrators. UPDATE: I found a better way to check a variable for null: Thomas Maurer. Powershell: check variable for null. It is like in javascript, just put it in the if-statement to test if it exists or not: