It's no fun to have speculators buy on rumor and sell on news. There are a few different ways to approach that. Announcing things after they are complete is certainly one way of providing good information by news. Unfortunately some people automatically sell on news, whether there has been rumor or not.

Another way is to have a constant status or stream of updates so that there is not rumor, just steady progress. When a milestone (without a date tied to it) is hit, then the status is updated to reflect that. One status method that might work is to have a roadmap of things that are being worked on, and provide updates every 2-3 weeks on progress, perhaps using a trello board or something similar.

The advantage of having a steady and open update is that the community is able to help in a more coordinated fashion, and although some of the community help might be off message, probably 80% or so will be useful and helpful.