Get Your Gear Ready Before Rehearsal Time
If your rehearsal time is 7:00 PM, you want to be in the rehearsal room at 6:30 or 6:45 PM to plug in your amp, get your guitar out of your case, tune up and — if you have pedals — get your pedal sounds turned on and dialed in. That way, others don't have to wait for you. At 7:00 PM, everybody should be ready to play.
Have a Purpose for Each Rehearsal
Decide beforehand what you all want to accomplish at your next rehearsal to make it more productive. For example, you could plan to make the section transitions of a particular song tighter before you record it next month, or create an ending for the live version of a song. Maybe you want to decide on a key for a song in-the-making, or sharpen your improvisation skills by jamming at the start of your next rehearsal.
Know Your Parts Before Rehearsal
Rehearsal is not the time to be having memory gaps with lyrics, fumbling your way through a solo or drum fill, or playing a shaky chord progression. Those are things you need to address at home during your personal practice time in between rehearsals. During rehearsals, you want to be free to deal with group-related issues like playing tight together; using dynamics; developing song arrangements; singing backing vocals; jamming, or trying out a set list.
Energy-Sappers to Avoid
-Instrumental noodling (especially when somebody's talking)
-Volume wars (where everybody gradually turns up)
-Endless chatting/discussions (save that for band meetings outside of rehearsal)
-Learning your song parts (do it at home)
-Insisting on playing instrumental parts that clash with the lead vocals or other instrument parts
Some might add emotional/romantic drama, drugs and alcohol to the list... be an adult and use your judgment.
Record and Videotape Your Rehearsals (and Shows)
Always record your rehearsals and shows. Videotape them too, if you can. Listen back and watch for things like:
-If you're all listening to each other
-If you're rushing
-If the key is right for the vocals
-If the tempo is the best one for this song
-If any new elements should be introduced as the song progresses
-if a song is sagging anywhere and needs to have something cut out
-If a part is being overused
-If unison parts really are played in unison
-If the song or set list has dynamics (or is everything played at the same volume level?)
-Too much dead time in between songs (you don't want that when playing live)
- If a solo's too long...
... and so on.
Rehearsing to a Click
Being tight isn't just a drummer's responsibility; it's the whole band's. If you're having frequent issues with rushing, sagging or other unintended tempo fluctuations as a group, it can be very helpful to rehearse with a click track as a group (and with a metronome during your personal practice.) Plug in a metronome or drum machine to the PA system so that everybody can hear the click through the speakers. Playing to a click will feel frustrating at times, but it'll make you rock solid as a band.
Note: once the group has a solid sense of time, you can intentionally play around with tempo fluctuations to build a song's energy, such as rushing just a little bit right before the chorus. But this has to be a collective intentional thing, otherwise you'll have problems during recording.
Listen to the Other Musicians
You will ALWAYS play and improvise better if you listen more to the other musicians than to the chatter in your head. Just let go and listen to them.
It's amazing how we all can forget something so basic. If you forget to breathe, it tightens up your body and affects your playing negatively. For singers, it's absolutely crucial to breathe in between lines or before long phrases or high notes, or your voice will wear out (this can be easy to forget if you're a singer who also plays an instrument).