When I was younger I played in a number of "Battle of the Bands." This usually meant my band would be playing with another 6 or 7 bands, and the organizer would ask us to share some of our gear. As a drummer, this really meant all my gear. Guitarists may be asked to share amps, but that's about it. At first I'd rarely agree - I mean, it's my gear - just mine. As I got older though, I've begun to change my mind on the matter.
At The Real School of Music, we have amps that you can always use when you play here. We have a few Fender Deluxe Tube Amps, and some Vox Tube Amps as well.
What you're worried about isn't the sharing; it's what happens if your gear breaks in the hands of another musician. This was a true threat to me ten years ago, but after a decade of beating up my own instruments and them never breaking, I'm here to tell you not to worry too much about it.
The worst that will likely happen to a drum set is that a head will break, or maybe your sticks. If you're worried about the sticks, ask each drummer to use their own. As for the heads, the only drums that really take a serious beating are the bass and snare. To help preserve the life of your bass drum, consider a patch to place where the beater hits. They make them for single and double pedals. The great thing about these impact pads is they extend the life of your bass drum head by months.
To help protect your snare drum, which for many is the most expensive drum, simply ask everyone to bring their own. You can also explore heavier heads, but you begin to risk the tone of the drum. If you're really worried about it, just ask all the drummers to bring their own snare - they all have them!
On the flip side, if you're the one borrowing another kit, consider the following:
Get an idea of the gear they have. Know if you're playing a quality kit or not. Additionally, if you're picky about your sound consider taking your cymbal bag. No need to take the stands - use the ones there. Also take your snare; you want your snare not only to respect the other drummer's equipment, but it's nice to play on familiar drums.
Thank the person lending you their kit. You might know them, you might not. Either way, them donating their drum set means that you don't have an extra hour of load in/load out time.
For you guitarists, you may at some point be asked to share amplifiers. Some amps - mainly tube amps - are a little fragile. They have moving parts; the tubes can come loose and also break. I know that even today I'm weary of lending my amp away unless I really trust the person, and if you have expensive gear, it's okay to politely decline sharing it.
If you have a solid-state amp, it's likely very strong and can take a knock. When you do share it, knobs are going to get turned and your settings will get messed up. No problem. What I do is take a photo of the settings with my phone, and make it my home screen image. That way, if someone tweaks the settings, I can jump back to my own in a heartbeat.
The same principles apply here as with drummers; respect the gear. Makes sense, right? Bring your own guitar and you own cable. Remember picks and don't forget to tune! Along with these principles, make sure you:
Get an idea of the gear they have. Know if you're playing with a nice amp or not. If you're not, consider bringing your own, or offering to share it instead. Also, many amps have a decent clean sound but can have horrible distortions. If it does sound good clean, invest in a nice distortion pedal. We love Celestial pedals. They aren't cheap, but you'll never need to buy anything else.
Thank the person lending you their amp. You might know them, you might not. Either way, them donating their amp means that you don't have to lug your monster around.
There are a number of benefits to sharing your gear. The time saved makes it worth it, but it all comes down to what you're comfortable with. You should never be afraid to day no to sharing, but consider the reasons why you'd prefer not to share it and weigh the pros and cons.
Have you been asked to share your gear before? Have you ever had a problem?