Best way to flush gearbox?

I am planning on flushing the old oil from my gearbox. Is it enough to just drain then refill then drain and refill again? or should I drain, refill go for a drive then drain and refill again?

I got my self a new centre bearing (finally) and a second hand Npower turbo back system. Going to do it all at once using a mates hoist :br:

 

They flush auto transmissions, not very often a manual unit.
If you have metallic particles in your oil when you drain it, it may be more appropriate to repair it rather than flush it.

 

Empty, Refill

As Graham says, if there is metal junk in there, no flush is going to fix the problem, no matter how much you try.

A good oil like Redline MT90 may help those worn synchros shift a bit smoother though

 

You are likely to make it worse if you flush it, Just change the oil to MT90