A couple tips from Tony Meazell (an excellent strength coach)
Maxim # 2: To lose muscle, you must temporarily stop training it.
Yes, there are ways to train muscle and minimize (not stop) hypertrophy, but if you're serious about losing mass, you must allow for (gasp!) atrophy to set in. That means not only stopping any direct work for at least a month, but also being very observant of any indirect tension you may be placing on the muscle. So for instance, if you want to lose mass from the trunk, this isn't a good time to do powerlifting. If you want to reduce your thighs, be careful of the amount of work they get from aerobics.
Understand that there's a difference between detraining — the physical and hormonal consequence of complete cessation of training — and specialized atrophy. So long as the usual level of tension is maintained for the rest of the body, detraining shouldn't be a problem. With athletes who like to do high intensity work, which usually innervates all muscle groups, I generally advise focusing on machine and isolation work for the other groups during the planned atrophy period. This maintains strength levels while minimizing inadvertent work.
Maxim # 3: To lose muscle, you must reduce calories.
Muscle is a tissue, which requires energy for maintenance or growth, and it'll decay in a hypocaloric environment. If you avoid training a muscle but eat enough or too many calories, the result might not be a reduction in muscle tissue but a reduction of resting muscular tension, commonly called tone, that will leave those big thighs every bit as stocky, but now soft and undefined.
But just as I wrote before, specialization doesn't require nearly the same caloric change as bulking and cutting. A reduction of about 250 calories a day should speed atrophy while shrinking some fat stores but minimizing other muscle loss.
Maxim # 4: You must eventually resume training.
As indicated above, atrophy and decreased muscle tone happen together. Assuming you don't like a soft, flabby look, you must eventually resume training that muscle.
But certain rules apply. I totally agree with other strength coaches that to minimize hypertrophy, volume should be kept extremely low, both in sets and reps. As a general rule, I advise trainees to perform no more than nine total sets a workout for the problematic body part, usually in sets of fewer than four reps. A side effect of this high-load approach is that muscle tone is at its highest level, and strength is preserved in such a way that a trainee isn't embarrassed by a lack of such in that area. My advice is to resume training at no more than 90% of previous RM, with 80% being better for many athletes.
They got bulky from over-doing squats, which I have cut back on.