Before The Rehearsal
You should learn to play yourself alone all the parts or songs you have planned for rehearsals, and along with metronome. Be self-critic, to avoid making mistakes that you won't be able to fix on the spot later on. Prepare all your ideas and write them out or record them, and even send them to your band peers, so they could be prepared to what you're about to work on. If you have some solo spots, make sure that you don't look for notes or options of how to play them on rehearsal. That really should be done at home. Make a loop of chord progressions you have to solo on and hit it. The only exception to this should be a complexly constructed solo that involves all band, though, that can be worked on alone, too. If you have in plan to make new songs on rehearsal, get enough sleep and try to warm up before, if you have time. Don't be late for rehearsals, especially if you pay for rehearse time, have respect for time of everyone else in the band, and they will respect yours. Also try to make sure you take care of all important phone calls and potential distractions before the rehearsals, as well as other basic stuff such as changing strings and similar things. And of course, don't go to rehearsals drunk or under some other influence.
During The Rehearsal
The worst thing that could happen on a rehearsal is when someone comes in really bad mood. It reflects on everyone else, and the music itself. If everything irritates you, find real reasons for that, and calm down. Don't snap on your band peers, because they might not be the source of your bad mood. Negative energy issue can be a real band-breaker.
Set up your sound. I suggest you to study the physical nature of sound of your instrument, and learn how to make a healthy sound for rehearsals. Don't allow yourselves to play in the noise. And make sure you don't play too loud. If you don't hear yourself well, and you hear that you're in balance with drums, lower other instruments, or fix your frequency conigurations. Sometimes more noise occurs at bad equilizer settings than on loud playing. Be informed about making a good sound setting or have someone set it up for you. Have this problem fixed in the beginning because it will drive you to deafness and uncreativity.
When you're working on your new stuff, be open to everyone's opinion, even if your're about to play the song you wrote yourself. Have everyone put a part of themselves in the song and listen to everyone's idea, because they just might provide a bit to the song that you might have missed or hadn't thought of before yourself. When some band members have to work on the part they play together and that doesn't involve you, don't make noise with your instrument, you'll distract them a lot, and it can get on a nerve easily. Rather think about other ideas you can provide for the song.
Always. ALWAYS stop whatever you're doing if you get some really great idea, and write it out or record it immediately, because no matter how good it might be, there is an enormously big chance that you will forget it. Don't allow yourself that. I lost few of ideas that way, and I totally regret it. It is also advisable to record your whole rehearsal in some way, especially if you're making new songs on the spot. You might find lots of potential material on those recordings.
When you fully practiced out a song with your band, and it is technically ready, don't stay on playing it while stand and not move at all, or even worse, not move and only look at your guitar. Jump, move, dance, give your peers a deadly eyelook, play with your guitar behind your back, play with your guitar on the floor, play with your teeth, with your tounge, whatever. Just don't stay on playing it with your mind. You should get to the point where you actually don't even think about what you play. When you get to playing live, there is a certain problem that comes out if you don't listen to what I just wrote, even more often if you play energetic music. The problem is that when you play, you turn the crowd on, and when you turn the crowd on, they turn you on even more, and you wanna do all the things you should've done and practiced on rehearsals, but you won't be able to do them, or you will do them, but make tons of mistakes. Turn of your lights and get a small light show for your rehearsals. Feel and go into it like you're on stage, in front of thousands of people! You'll find this very interesting and amusing, and it will spare you of all the bad things on the stage. Even make a small choreography of your live shows if necessary, and go into details. See what might happen on stage, and try to work it out on rehearsals, before unwanted consequences can occur.
I also advise you to squeeze all the best from the rehearsal time. Don't go in senseless jams (unless you know for sure they'll benefit your bandwork), don't have a cigar every 5 minutes, don't get stuck in some non-music related chats for too long, don't have too long breaks (though, they are useful sometimes, when you go way over the top and it really doesn't make sense to push yourself too much, because nothing creative will happen) and most importantly, don't wander off with your thoughts. Be there 120%, or at least 100%, and you'll notice the difference.
After The Rehearsal
If you have the time, go for a drink with your peers and talk about everything you've done on rehearsals. Review all your ideas, and talk about what you could do next. Give yourselves some directions for next rehearsals. Fix all the other problems you have in between yourselves right here, and not during rehearsals. And have a good time, establish a great personal relationships. If you enjoy being with your band outside the band, you will enjoy it even more when you're in the band. And everyone who'll hear or see the band will notice that, and it plays a big factor of success.
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