What is the best day and time of day to publish app (or its updates) to the Market? What have you found to work best and generate maximum number of downloads/sales?
16.22 GMT
That's what the folks at Firedroid determined for their game Barrr. This article is a pretty good read if you have a couple minutes.
We released at 16.22 GMT indeed. We didn't really research it beforehand, it just seemed like there weren't a lot of apps updating around that point.
If you're releasing it, download and rate it yourself immediately (and get friends to do the same), I saw some apps being released around the same time not do that and altough I can't support it with data I think it had some influence.
Besides that there isn't really a whole lot to do on the market and it really depends on how good your app is. Do make sure to have a nice description with some keywords you want to "rank" on in it.
Get your app on appbrain and more websites like it. Your app will be on there anyways because they scrape the market but you can add Youtube-clips and more screenshots, although google's developer backend supports those things now as well.
We've had some big jumps in our number of download caused by websites writing about it (gizmodo gave us a 20K+ boost), so try tipping a few of them off. We didn't really do that and still websites wrote about us, I think it's worth trying to get some coverage this way.
We don't update often and it only brings in about 1000 extra downloads, compared to our 300-700 daily average it doesn't really have a lot of influence. I'm not a big fan of apps that update weekly or even more often just to get in the "just in"-section, I think it's just rude to current users and abuse of the functionality.
It's more info than you asked for, I hope it helps.
The best time to submit/update an app is when most potential users are active but the fewest number of apps get deployed by other developers. To determine the amount of games that get published (and thus appear in the "Just in" list) I monitored the Android Market with a script for two months. I also looked at the time one of my games get started for the first time over several weeks to approximate the time games get installed.
Combining both aspects I assume that you should submit your app in the evening (GMT). In addition, publishing at the weekend is better than publishing on weekdays. However, publishing today is obviously better than publishing weeks later because users can only install published apps ;)