@Stumbo hogged all the glory on this one. The only additional things I can think of relates to distance and aiming.
Per the inverse square law, SPL drops off at a rate of 6dB for each doubling of distance. So when you are setting up with other players, this is an important factor to keep in mind that can help you hear better. In my opinion, we generally want to hear our instrument about 6-10dB louder than everything else in the mix when we are playing. Obviously on a small bandstand there is not enough space for everyone to get their 6-10dB, so you may have to settle for not hearing as well as you like, but trying to keep some distance between your amp and other amps that are putting out sound in the same frequency spectrum is a good strategy. This also relates to "masking and frequency slotting" which are terms you should Google and read about in the context of mixing audio.
Another piece of the puzzle. Low frequencies are omnidirectional, so the bass can be especially problematic. As the frequency increases, speakers typically develop an increasing amount of pattern control. So guitar speakers tend to shoot the sound out the front if they are sealed, or out the front and back if the cabinet is an open back design. The higher frequencies of the bass are also directions, which is why tilting your cab back to aim it at your ears can help you hear better. Basically aiming the cab at your ears allows you to hear the full range of your rigs frequency response, instead of having a significance range of frequencies shoot by below ear level.
Unfortunately many guitar players do not like to hear the direct sound coming from their amp, so they will crank up and aim their speaker away from their ears. The idea is to pump the entire room with volume so they are surrounded with a nice warm sound. Although this approach sounds really nice to the guitarist, it tends to make it difficult for other band members to hear. Either try to convince the guitarist to change his/her monitoring strategy, which unfortunately is unlikely to succeed, or find another guitarist to work with.
A few words about elevating your cab. With the cab on the floor you get near perfect acoustic coupling between the cab and the floor, which provides an increase in low end efficiency. This is called half space loading. As you elevate your cab some cancellation will occur that is related to the distance between the sound source and the boundary. The cancellation is centered at the frequency where the distance from the sound source to the boundary is 1/4 wavelength. Basically the sound travels from the sound source 1/4 wavelength, reflects off the boundary, and travels back 1/4 wavelength. Of course the total distance is 1/2 wavelength, so the reflected wave arrives 180 degree out of phase and cancels with the next cycle coming from the speaker. I usually like to elevate my cabs at least 18 inches, which gives some cancellation in the very low mids. You can learn more by Googling "boundary interference".
