Plain yogurt doesn't really have much of a taste once it goes into a shake, which is why people put it in recipies to add a creamy texture without overflavoring... anyhow... Try two scoops of lean whey, oats, and make your high GI carbs be a completely ripe banana. Mix this with water or skim milk. You'll do great. Another good carb option is berries, they are flavor dominators in shakes, big time.
Oh and pre-mixing would be a good idea since you'll want that shake about 20 minutes after your workout. Not sure what your gym situation is but most people would be hard pressed to get home, mix the shake, and drink it in the time frame.