The two variations that I've seen commonly used are these:
private Vector DoAdd(Vector vector) { ... }
and
private Vector AddImpl(Vector vector) { ... }
Neither one is particularly satisfactory, but they're what I've seen.
I've never seen a convention that ALL private methods should have a prefix - the mere thought of it makes me shudder!
It's bad enough dealing with all the C++ developers who prefix every member in sight with "_" - and I'm speaking as a former Delphi developer who used to prefix every member with "F". I'm still recovering from that!
