I know I just said it in the other thread, but do have a read of Fight and Be Right (link in my sig). It covers this in detail.
Basically, the best way to give the Mahdists a boost is by avoiding the death of the Mahdi in 1885. This is easy enough- just ensure he doesn't get typhus. Then the Mahdists don't suffer the squabbling over the succession that they had IOTL, and can get back to their jihad a bit quicker. Ultimately however, the more successful the Mahdists are, the more likely somebody, probably the British, will step on them early.
Best case scenario for the Mahdists, I think, is that the British get dreadfully distracted elsewhere, decide not to bother with the Sudan for a bit, and finally get round to crushing the rebellion sometime in the 1900s, Somaliland/Senussi style. But that needs Gordon not being killed, and he was so determined to martyr himself that this is a surprisingly difficult thing to do.