Last Tuesday me and my ensemble (www.barbercue.se) had the privilege of being coached by Anders Edenroth of TRG. This was quite a mind-opening experience on how to rehearse effectively. Instead of focusing on technicalities or trying to adjust individual mistakes in timing or intonation Anders used a set of exercises that made us open our ears.
When practicing and performing acappella you easily end up focusing all your attention to your own part (quite naturally since singing acappella can be quite demanding technically). However, when five singers sing an arrangement without really knowing what the others are doing the result will be at best... nice. Even if the individual parts are sung well you will never get that groovy, funky magic if you don't sing together.
One of the exercises Anders made us try out was to let one singer at a time adjust the tempo freely while the others should try to follow. This way you must constantly focus all your attention to someone else instead of yourself. The same exercice was applied to intonation.
Another interesting comment Anders had was on the way we sing supporting parts (e.g. harmonizing a chorus with the melody). Once again the bottom line was: "listen to the others". If you subjectively regard your own part as the most important one the audience will not percieve a melody that stands out of the arrangement but rather five parts that are hopefully somewhat related. When singing acappella you must always make sure the melody is delivered to the audience supported, not obscured, by the other parts.
In the end some advice was given how to improve those sections of an arrangement that always sound bad. Transposing the song to a neighbouring key, singing on just a vowel (e.g. aahh), changing the lyrics, add one part on the piano as reference etcetera were some useful tools Anders gave us. The basic thought of all these approaches is that once you have taught your vocal folds and your mind the "mistakes" you need to drastically change something in order to correct them, otherwise you will keep falling out of pitch each time you reach that tricky section.