Kill minor game characters for no good reason. Seems to work wonders.
I've always found that it's a bit of a balancing act in gaining influence with Mandalore, HK47, G0-T0 and Hanharr (on the rare occassions that I have used him).
There may be a few minor spoilers in the following:
When he is talking about aiding the Republic, ask about giving something to help - when he offers credits to you personally, choose the most aggressive response. Should be along the lines of 'I'll kill everyone in the way' or something. That'll start you off. Then you'll probably have to do one of the killings that gains dark side influence. After that, you should be able to ask him about operations on Nar Shaddaa. I find it is best to mention Vogga then choose the reply that is nicest to the Republic - though not the 'I wouldn't wish to harm the Republic for my own gain' type one - he seems to take that as sarcastic...
That should more or less do it.