Klingons: I default to ridges-Klingons regardless of era, unless specifically stated in the text that we are dealing with one or the other.
Trill: Always DS9
Cardassians: DS9 always unless specifically I'm reading about Gul Macet then I picture him as he appeared in TNG.
Andorians: ENT style always. In this case even if specifically mentioning the other style, I override that in my mind and go with ENT, unless it plays some special part in the story itself.
Tellarties: ENT -- but that's because that's my only remembered exposure to them and don't know what they looked like elsewhere.
Tholians -- I don't picture them as they appeared on screen at all. I picture them more like N'Grath from Babylon 5
but with smaller eyes.
I also have a stereotype about Humans I picture in books too. I go with what is described about them up to a point. For example I have a great deal of difficulty remembering to picture Renal Keru in Titan as a big burly bearded man. I usually start out with a base of average height, and average Hollywood-weight (so much thinner than normal), and ranging in average to well above average looks.