Allow the gear to warm to room temperature after bringing it from a cold environment. This protects the tubes from cracking and prevents electrical problems due to buildup of condensation.
If you have especially vintage gear, power it up gradually with a variable AC power supply (Variac). This protects the tubes from the jolt of suddenly getting full power.
Use standby mode instead of switching your deck on and off all the time. This protects your tubes and keeps them warm and operating in a way that is gentler than being switched on and off.
Allow the tape deck to warm up for about an hour before you start recording to get the most consistent sound. It takes that long for a tube circuit to stabilize.