Romulans I tend to imagine TNG/DS9 style.
Klingons: As appropriate for the time period and the book.
Andorians, Tellarites, and Tholians: ENT, definitely.
Cardassians: A mix of all appearances, actually. I try to forget those awful brown uniforms, though--but I kind of like the idea of there being a diversity of ethnic groups.
Borg: I tend to imagine a combination of FC and TNG features...it's a little hard to explain, except for looking at the way Locutus was portrayed in FC.
Trill: Definitely DS9.