The option that I tend to use, mostly because it's simple and I don't have to think, is using Write-Output as below. Write-Output will put an EOL marker in the string for you and you can simply output the finished string.
Write-Output $stringThatNeedsEOLMarker | Out-File -FilePath PathToFile -Append
Alternatively, you could also just build the entire string using Write-Output and then push the finished string into Out-File.