=> Do not download multiple apps at once from the market. Even two simultaneous apps can screw your phone up.
=> Never Use the "UPDATE ALL" feature from the market. Update each app separately.
=> When you flash a new ROM, restore your apps using titanium backup before adding a Google Account. This will prevent the market from restoring all your apps at once.
=> NEVER EVER PULL THE BATTERY This is very important. Pulling your battery out is the most prominent cause of eMMC bricks. Use alternatives like -
=> Use ADB Commands to reboot into recovery
=> Press the Volume up, Volume Down and Power buttons together to reeboot the phone. In a bootloop, take off the battery cover (without pulling the battery) and press the buttons. The phone will turn off for 2-3 seconds before it turns back on. That's your window. Pull the battery in this time.
=> If you absolutely have to pull the battery, re-insert it at least 30 seconds later.