How to keep the good maintenance of any brand of a PLC esp of Siemens PLC like S 7 - 300 & S 7 - 200 ?

Just maintain Temperature of PLC cabinet and regular use blower to remove dust particles specially in rainy season. Also check Power Supply as Mahdi explain already. from software side take backup in every month and whenever you modify PLC program add symbol and comment.

The PM for this type of PLCs are
-Change the back-up battery on time
-Ensure that the filter installed on the cabinet where these PLC are installed is cleaned every three months to ensure the geed ventilation.
-Take back up copy of the application program of each PLC every three months or whenever changes modifications are carried out.
-Ensure to connect the main supply 230VAC for the PS of PLCs via UPS
Otherwise these types of PLC can survive different type of environments

Check your line voltages, especially in summer. Just like right now where it is hot and using power to the max. I have seen it drop below 100 VAC when it should be 105-125 VAC. If the voltage drops too far the equipment should be shutdown. Constant Voltage Transformers and having a good Power Factor help. If other equipment doesn't have voltage stablizers then it could be a safety hazard.

Use a wet vacuum cleaner to suck out all the dust, metal filing and chemicals. Inspect wiring, missing lables, contacts and burnt components. Do this when the equipment is shut down at least every 6 months. Have a spare set of filters, so that these can be replaced often while the machine is running. Use the aluminum type where you can blow them out. Just like a furnace filter, it all depends on environment. Don't blowout any cabinet, the particles just go other places in cabinet.

Contact your plant engineer or maintenance department. You might want to find out if anyone does good electronic maintenance technician work.

4 things
1. Incoming supply of should be regulated by Voltage stabilizer/ UPS or at least transformer.
2. All wiring should be tight & firm
3. Temperature should be controlled
4. Keep electrical panel dust free.