In winter, leave amp on standby until it gets hot before you play it. Wait until all condensation has evaporated from the metal chassis
Make sure that the loud-speaker is connected before turning the amplifier on.
When first turning on the power, listen for sound from the speaker before blasting a power chord. Turn up the preamp and listen for white noise. On an old JMP Marshall turning the presence control will make a scratching sound. This is the method that I like to use to assure myself that the the cabinet is plugged in
On old JMP's that have a pull-out impedance selector plug, check the speaker impedance selector while playing at very low volume to see if the sound cuts out as you move it.
Check the speaker jack on the loud-speaker cabinet to make sure that the sound does not cut-out when you move the cable.
Never block air circulation by putting a jacket on top of the amp while it is operating.
Don't put drinks or beer bottles on the amp. If someone else from the band insists on putting their glass or bottle on your amp, thank them and chug their drink as fast as you can. That should cure them.
Test for electric shocks from the microphone before before singing. Using your tongue to find out is never a good idea. Using a wireless system or a wind-screen on the mic will prevent electrocution.
If you have an old Fender combo check for power tubes that may have fallen out.
If the amp head suffers from microphonic problems at louder volumes, put the amp beside the cabinet instead of on top of it.
Radio Shack sells Fender strings, fuses, and AC cables (maybe even picks and a guitar strap). There is usually one in every town.
You can obtain fuses from an automotive repair garage.
Repair torn speaker suspensions with Jiffy-Sew, patch larger holes with tape until you can get it re-coned.