For myself, I normally rely on a spot meter (mine is the Zone VI modified Soligor Digispot II that I have used since 1988) for my measuring for transparency exposure. Understand that most transparency film only has between 3 1/2 to 4 1/2 stops of latitude. As has been mentioned, for transparencies expose for the highlights.
When I measure the light I first measure the exposure range, finding the darkest and the brightest parts to a scene. Once I know if the film falls within the exposure range of 3 1/2 to possibly 4 1/2 stops, I decide if I will record my initial composition or make some adjustment to stay within this range. If the exposure range is within these constraints, I will usually adjust my exposure opening up the aperture (or combining the aperture adjustment with shutter speed) by 1 1/2 stops. This is the same idea as exposing for snow or white sand so that you will hold detail in the highlights and not end up with "muddy" snow or sand (remember that a meter will read anything that it is "seeing" as 18% reflectance for a reflected meter; an incident meter also reads for 18%).
If the scene does not fall within the 3 1/2 to 4 1/2 stop range, I will either recompose the image or select a graduated ND filter (gray at the top grading to clear at the bottom- suggest using rectangular type) if they can be used in the situation. If using graduated ND you will have to decide on the intensity of the gradation, normally 1 to 3 or even 4 or 5 stops (these can be combined if need be) and the kind of transition from soft to hard). Also, Singh-Ray makes the special reverse ND graduated filters which are darkest gray in the center grading to clear at the top on one side and clear on the other. These special filters are used when you are in a situation for instance at sunrise, looking into the sun where the horizon is the brightest part of the scene.
Rich