Invest in good sound engineering. Unless your group's music is entirely acoustic, you will need good quality equipment and proper configuration of your microphones, power equipment, amplifiers and mixers. Even if your music-making is perfect behind the microphone, bad engineering can still ruin the show. If no one in your group is knowledgeable about mixing and mics, hire a professional at least once to give you a quick lesson.
Choose the right music to perform. Whatever type of music you play, it needs to be of good quality and a proper match for your group's players and their abilities. Don't play music that's too hard for you to master as musicians, and always be willing to objectively assess the quality of any original music: Could your skills as composers be what's holding you back, rather than your playing ability itself?
Arrange a good combination of musicians in your group. The reason certain configurations of instruments and vocalists become traditional is because they have a balanced sound when playing together. If you know your ensemble's lineup is a bit less traditional, think of what musicians you might want to add for a better sound. With popular music, this usually means a good balance between rhythmic instruments (like bass and drums), melodic instruments (including woodwinds and lead guitar) and chord-playing instruments (such as the piano and rhythm guitar). In classical playing, the balance of instruments usually becomes an issue if a large ensemble has too many people in one section and not enough in another.
Work on intonation. Start every rehearsal with a careful tuning of your instruments, making sure they align with one another. If there are instruments in the ensemble with fixed tuning (such as a piano or mallet percussion), tune to these, since they cannot adjust to you. Throughout the rehearsal, listen to how well you are matching each other and make adjustments. A musician's ability to hear and correct intonation improves the more she pays attention to it, and good intonation cannot be ignored by any musician who wants to sound like a pro.
Tighten up your playing rhythmically. Not only must everyone in the ensemble be playing the correct rhythms, but the manner in which those rhythms are executed must line up. Improving your sound in this area requires listening to one another carefully, but can be helped with the use of a metronome during practice.
Take responsibility as individuals. The work you do together during rehearsals can only help so much; each individual musician must work on his own to improve his playing and minimize errors.