Simple and appropriate:

A guid New Year to ane an a' And mony may ye see
(A good New Year to one and all And many may you see)

Fancier (and a might jingoistic):

Here's tae us. Wha's like us. Damn few, and they're a' deid!
(Here's to us. Who's like us. Not many, and they're all dead!)

Depending on the gathering you may hear other "toasts" such as:

Gaun yoursel', Big Man!
(You're a big chap, drinking a lot and are going to continue to do so!)

Gie it laldie!
(Party furiously!)

For our First Footers, here's an old Scottish saying about coal:

"Lang may ye're lum reek wi' ither foolks coal"
(Long may your chimney smoke with other folks' coal)