I have had the opportunity to study chamber music with several different teachers and had both good and bad experiences. Here are some of the things I have found to be key in making a good chamber music group sparkle!
Ensemble: Playing together at the same time is great but if someone is playing staccatto and another person a bit longer, it's not tidy. Try to match lengths of notes and articulation. Vary the articulation to acheive good phrasings and colorings to suit the piece. If it's stringed instruments, make sure that when suitable that the bows are together and are going up and down at the same time and that the shaping of the strokes are created similarly.
Dynamics/Colorings: Dynamic levels are not only created by intensity of sound or backing off of sound but things like if you use vibrato or not, or if you use a lighter stroke/note length as opposed to a longer stroke/note length. For strings, they can move their bows to different sounding points to create different colors and that in combination with vibrato can make a note either die or intensify. Aside from that, the members of your chamber group can make their body movement make the music intensify or relax.
Group: Communicate! Make sure your groups communicate! Communication is what makes it possible for everyone to be relaxed and comfortable to be able to take liberties in tempo, and know when/how to do dynamics. It takes that metronomic edge off to communicate and it makes the audience relax to see a group looking at one another and really communicating.
Above all, have fun! Chamber music is a great journey in music making and friendship. When it comes down to it, if you aren't having fun the group isn't going to sound it's best. In performance forget everything you've rehearsed and just have fun and it will make things more relaxed, enjoyable and make your performance sparkle.