More frequent, perhaps smaller, updates.
As one of the people who (quite happily) donates (not a fortune) to Tarn for his work, I have to say that I lose interest when waiting so long between releases.
I realize he's working on something huge, but the fact remains that some obnoxious bugs have been sitting around in the code all this time during development. Also, it's really going to become a problem in the future (the near future, I suspect) if the interface isn't turned over to an API and the development of the interface given to the community.
If Tarn continues to take the approach of tackling huge development cycles with no intermediate code releases that alleviate bugs and the interface continues to be ... not up to the standards I'm sure Tarn would like ... the community is going to eventually give up on the project.
I say this and urge a different development cycle (moderately different only) wherein a week on, week off approach is used. The major new features could be worked on every other week while in the intervening weeks minor features and bug fixes (hey, some code optimization would be nice, too!) could be released. The major development code would be disabled (perhaps some of it could simply be commented out at compile) until he's ready to release it to the wild.
I think this would keep the sense of 'something new' always coming out and the feeling of interaction with Tarn at a high. One of the great things about DF is to be able to point out a bug or possible feature improvement (or addition) and then see it added/fixed relatively quickly.
Which goes back to the interface API. If the interface could be handled via an API, then there would be a new stream of updates and improvements to the game which Tarn would not even be responsible for. Thus generating the 'something new' feeling, the community involvement aspect and addressing what is probably DF's single greatest weakness.
I have listened to all of Tarn's interviews that I'm aware of and I have read everything that I have had access to that he's published. I realize that he has good reasons for not having dealt with the interface yet, but as he continues to add complexities to the game it will become a serious problem. The army arc alone leads to untold difficulties with the current interface and military management once the player involvement side is handled.
So that's my 2 cents.
And for what it's worth, I'll be donating again when I get my next paycheck - I may feel like I lose interest in the time during major development cycles, but I want the development to continue - preferably with a different tack, but any process is better than none!