I've never been great at this, but as I get older I figure it's one of those crucial fundamentals that I should focus more on.
Plus manuals, in my opinion, just feel dope.
I find pushing the nose down further than I think I should has helped a lot. This is probably just the commitment factor, but it seems to be easier to balance that way.
I've been doing more strengthening exercises - legs, and core - mainly to reduce the risk of injury/speed up recovery but as a side-effect those exercises have helped out a lot not only with nose manuals but also manuals in general.
I agree with what a previous poster said that nose manuals do seem to be easier on Indy's, but that might be due to a lot of different madness inducing factors that I don't really want to think about - they seem to have a lot more slack/room for error.