I typically prefix private member variables with an underscore.
It just makes them easier to spot when you're trying to read the code and it's allowed by the Microsoft Guidelines:
public class Something
{
private string _someString = "";
public string SomeString
{
get
{
return _someString;
}
set
{
// Some validation
_someString = value;
}
}
}
Like others have said, the more important thing is to be consistent. If you're on a team that has a coding standard that does things the m_ way, don't try to be a rebel and do yours another. It will just make things more difficult for everybody else.
