I keep getting stuck in Safe Mode!
This usually means that one of your installed packages is broken or misbehaving in some way. Follow the instructions on the Safe Mode help page to solve the issue (including uninstalling recently-installed extensions, checking for iOS version compatibility, and making sure your packages are from trusted sources).
If you paid for a package that consistently is not working, you may want to email the author using the contact link provided on top of the package's description page in Cydia.
How do I keep my device safe and using minimal battery?
Use the same caution when installing packages from Cydia that you'd use when installing software on your desktop computer, including researching first and choosing software from reputable developers. The community repositories (BigBoss, ModMyi, and ZodTTD/MacCiti) keep an eye on the packages they distribute, but ultimately it is up to you to decide what to put on your device.
Most extensions by experienced developers add little or no battery usage. Often the best way to save battery on your device is to turn off WiFi and 3G when you do not need them for a while. Other than that, you may see a bit of increased battery usage from extensions and tweaks that use the internet frequently (such as ones that check for mail, check the weather, or activate a personal WiFi hotspot) or change the screen frequently (such as video wallpapers and animated lockscreens).
How do I fix these annoying errors that pop up while using Cydia?
If you've just installed Cydia on your device and you see "cannot locate package" errors, close Cydia and open it again, and then wait for it to finish downloading and loading data.
If you see "untrusted server certificate" or "an SSL error has occurred" preventing you from using Cydia, check to make sure your device's date is set correctly (using your device's Settings app).
If your device finishes reloading data and then pops up a black screen including red and yellow errors, they usually indicate a problem with one of the repositories you have installed (or sometimes one of the packages). Cydia is still working fine, but you may want to try refreshing data again or removing the broken repository. Keep in mind that Cydia has no control over third-party repositories. Common repository (source) errors include:
"Failed to fetch": This usually happens when a source does not exist. You may have typed in the wrong URL, the source may have shut down, or the source may be broken. This is often a temporary problem, so wait a few hours and try reloading data again. If it keeps happening, look carefully at the error message to find the URL of the broken repository, and then you can choose to remove that repository by going to Sources and tapping Edit. If the source can't be removed that way, it may have been installed as a package, which means you need to uninstall that repository package.
"POSIX error: Operation Timed Out": Make sure your device has access to the internet and try again. If this error keeps happening, it is likely caused by a repository server being temporarily offline. Try again later. You can also choose to remove the repository (see the method above).
"GPG error": This means that a repository is configured incorrectly. You can usually safely ignore this.
If I use Apple's Erase All Content and Settings option, why does my device not boot?
On jailbroken devices, using the "Erase All Content and Settings" option in your device's Settings app will cause your device to require a full restore. :(
This happens because during the process of jailbreaking, your jailbreaking tool reorganizes some of the files on your device in order to make room for Cydia and other jailbreak-only files, so the "Erase All Content and Settings" option ends up erasing important files instead of just settings.