We'll assume you have a general "theme" for the evening, so you should design your set accordingly.  You can do a lot with set pieces and lighting to keep the show from looking too static.

As far as the musical numbers are concerned, I assume that you will have solo pieces and some choral or ensemble numbers.  Some slow tunes and some up tempo.  And some male singers, and some female singers.  Mixing them up will go a long way toward keeping the evening feeling fresh.

One thing you should definitely do is make sure you have different "personality" types among your solo performers.  They shouldn't individually be too similar in physical appearance, vocal range, age and attitude.  Have a cynical man (or woman) of the world, a sultry siren, a leading man type, a young boyish (or girlish) optimist, a dramatic type, a comic type, etc., etc., etc.  I hope I'm explaining myself well enough.

Think of it as a variety show.  A vaudeville evening with a something for everybody approach.

At least, that's the direction I'd take things.

One revue I took part in chose 5 Broadway shows and did three numbers from each show (2 features - solos/duets/trios - and 1 choral number). It was a type of broadway over the past 50 years thing so there was a "narrator" or emcee who would come on and say something about the musical being showcased and the year it debuted. The audience loved it!