First of all..play what you want to play,and find the audience that will enjoy the same music...

That being said, we are losing the audience that relates to the 20's to 40's music...in general..It doesn't mean we can't play the super well known songs from that era...for example Gershwin,Porter,Kerns etc...
My personal preferrences...Late 30's[songs from film hits]...Bigband from the 40's, and I definately add the 50's[great hits in this era], I also like to play the 60's tapering off after the mid sixties[Humperdink and Elvis are covered as are Bobby Vinton,Vee and Rydell...
If we can't keep an audience with this material..send them to the Karaoke bar..

Naturally we need to have a few of the Disco/dance hits of the 70's and some Motown from late sixties.. throw in some novelty dance tunes from the eighties...and you've got it...Forget the 90's and newer[that's 1990's..]..