Here's a script that's very useful for me -- remove old backup files (where I'm defining old as more than a week old).
$dt = (Get-Date).AddDays(-7)
$dir = "D:\Backups"
Get-Child-Item $dir -recurse
| where
{ $_.LastWriteTime -le $dt }
| foreach
($_)
{remove-item $_.fullname -whatif}
Note that you'll want to remove the "-whatif" to actually delete the files, rather than just show them in a test mode!