While the Android Market has many a great quality, I (and many other users) have noticed some reliability issues as of late. Here's how to update your apps when the Market starts throwing you errors.
Recently, I've started getting a lot of errors when I try to update my applications in the Market, specifically "Download Unsuccessful" or "Installation Unsuccessful". The problem with this is that it doesn't give you any information beyond that error, when there might be a few different things causing it. After searching around the net and dealing with about five different apps that refused to install, I've compiled a few of the solutions that worked for me. If you're getting errors from the Market, try one of the following to push those updates through.
Clear the Market's Cache and Data
One of the first solutions to this problem (and one that served me well for a while) was to clear the Market's cache and data. To do this, head into your Settings app and go to Applications > Manage Applications > All > Market and hit "Clear Data" (this should clear the cache as well). Go back to your list of apps and to the same with the Download Manager app, too. Then, go back to the Market and try to re-download the app in question. If it works, you're all set. If not, it's likely an SD card issue, in which case read on.
Unmount Your SD Card Before Updating
After I started moving all my apps to my SD card, I noticed this error began popping up much more often. The problem, it turns out, is that sometimes your phone has trouble updating apps that are installed to the SD card, even if they're apps that are allowed to be there (i.e. you didn't move them using, say, Cyanogen's hack). In fact, I've even had this happen with apps that automatically installed themselves on the SD card, so even if you haven't used App 2 SD, this might still be the cause of your error.
The easiest way to fix this is to just unmount your SD card before updating those apps. So, if you start getting errors, head into Settings > SD Card & Phone Storage and hit Unmount SD Card. When it's fully unmounted (it'll take a second), go ahead and re-update your apps from the Market. They should reinstall just fine (though you may have to search the Market for the app if it isn't on your Downloads page). You'll then have to re-mount the SD card and move the app over again using App 2 SD.
Alternatively, you can move the troublemaking apps from the SD card back to your phone using App 2 SD, updating them, and then moving them back to your SD card. Of course, you could just keep your apps on your phone's internal memory, just know that it may slow down some older phones.
Uninstall Market Updates
I haven't had to use this one before, but it bears mentioning: some users have found that going into Settings > Applications > All > Market and hitting "Uninstall Updates" fixes the problem for them. I can't personally comment on this myself (the button's always been greyed out when I try), but it's worth a shot if you're still getting errors.
Uninstall the App and Re-Install from Scratch
It's a simple solution, but oddly one I did not try until the very end because I didn't think about it — just uninstall the app and try again. If you've already tried to update it, you won't be able to uninstall it from the Market, but if you go to Settings > Applications > Manage Applications and uninstall it from there (by tapping the app and pressing "Uninstall"), you should be able to re-download it from the Market without issue.
Delete the Original APK (Rooted Phones and Google Apps Only)
The last method, which is specific to phones running custom ROMs and errors with Google apps (like Gmail, Voice Search, or Maps) involves deleting the original APK with Root Explorer. Google's started pushing updates to their apps through the Market, but phones running custom ROMs have trouble installing the apps, so you'll need to get Root Explorer from the Market, navigate to /System/app, hit "Mount R/W", and delete the app in question. Or, to be safe, just rename the app, e.g. change Gmail.apk to Gmail.apk.backup. Then, hit up the Market and install the new version of the app and everything should run smoothly from there on out.
This list doesn't include every single solution to the problem, but it does include the most likely solutions. There are a few other ones floating around the internet that I've never tried or needed (such as deleting the .android_secure folder and deleting the com.android.providers.settings file using Root Explorer), but the above tips should get most apps to behave themselves. I'm a little shocked at how many different issues cause this generic error, and a little disappointed in Android for letting errors this egregious thrive — but at least now you know a few ways to fix them yourself. Got another method that worked for you where the above didn't? Share it with us in the comments.