brief answer......less weights, more speed work......(hat tip to Jake the Mus from Once Were Warriors);
take a look at this clip from an excellent film (language & sexism warnings); http://www.youtube.com/watch?v=VZSv4HlyfIo watch it through till 2.00 min for the speed work reference.
(or for a slightly longer clip, if you want to see more http://www.youtube.com/watch?v=RCHe-0w9KX8)

longer answer......look for a sport (or sports person) who's physique you would like to "convert" to, find out how they train & do the same. you'll probably need to eat less as well.

a couple of local (Australian) examples that come to mind would be; Anthony Mundine, who went from Rugby League to Boxing & then dropped weight divisions http://en.wikipedia.org/wiki/Anthony_Mundine & Israel Folau who went from Rugby League to AFL http://en.wikipedia.org/wiki/Israel_Folau

you may be able to google some images for these guys, both went from bulky muscular types to more lean muscular type.

....another thought, are you taking any 'body building' type supps or powders. if so, may be worth giving them up, unless they are good for your general well being. & you'll save some money as well. Creatine in particular seems to be effective in increasing muscle volume (by increasing water in the muscle), so if you currently take Creatine, stop.

Working more on heavy weight with low reps (1-3 reps per set) will give you strong dense muscle. If you diet enough to lose weight while keeping your 1rms the same you should see a smaller body with the same strength. For the love of god don't abandon your hard earned strength.

If thats a real question, and you dont want to hear to stop lifting weights then: The best way to do this is to starve yourself.

Diet doing everything wrong.

No heavy resistance workouts (nothing heavier than your 15RM)

Excessive cardio

Low protein (~ < .5g/lb)

Low calories (~10kcal/lb)

reduce muscle? reduce protein intake, take thyroid hormones, do excessive cardio training, take synthetic cortisol, make sure not to get enough sleep, eat high GI carbs and let your blood sugar bunk afterwards, start boozing, eat lots of soy

^ lots of valid ways to reduce muscle ;)

Start doing 4 mile plus runs a few times a week and you can watch the muscle disapear. I know that seem to work for me especially combined with intermitent fasting.

If you heavily cut out carbs, your muscle glycogen stores will deplete and you will lose some water as well. This way your muscles might dwindle a little bit.

From an evolutionary stance, I would suggest that extreme muscular mass is not an advantage, since it requieres more energy to sustain and might be hindering for sprints, climbing etc. , so it could possibly happen that a strict low/zero carb paleo diet will shape you into a leaner version - but no necessarily weaker version.

You might want to try the Twinkie diet. Or become vegan.

I am a 24 year old female, and I also wish to lose muscle mass. I have competed in bikini competitions and have added a lot of muscle to my small frame with ease. (I enjoy moving heavy weight!) But, when I am not dieted down to be on stage, and am at a normal 18-21% body fat, I feel very large. I want to go back to being petite and feminine. I don't want to run miles on end daily, as I learn more about the detrimental health effects of chronic cardio. Any suggestions on how I can still enjoy being a very active person (and even hit the gym) but still manage to lose some of my mass (fat and muscle both)?

Try long-distance running. Seriously. There's nothing like running -- and lots of it -- for turning you into a wiry beanpole!

(Some people here are reporting they get big legs from it, but that hasn't been my experience.)

If you want to have lower muscle volume, then go keto (VLC, very low carb). After a while ( a week or so), your muscle's glycogen stores will deplete, and you'll lose volume. Any time you eat carbs, your muscles will refill their stores, and regain their volume. You can still do your weight training if you want, but you won't have as much "fuel in the tank" when you do your lifts, so you can forget about intense 45min lifting sessions, or at least that was the case with me.

Addressing the "why" - why do you want less mass? If you're concerned about standing out, then don't be afraid to stand out from a crowd of weak, undernourished people. Having muscle mass isn't bad.

Do body weight exercises for 10-12 minutes per day (High reps). Then jump rope for 20 mins.

I've lost significant muscle mass after I started Paleo + Crossfit. If you do metcons of longer duration, it might help you out as well. As pointed out by others, keep strengt work at low repetitions, and do speed work as well. Caloric restriction might further boost the progress as well.

Switch to forms of exercise in which bulk is a decided disadvantage: running, body weight/gymnastic rings, that sort of thing. Volleyball, particularly sand, tends to create leaner yet muscular physiques. Look at the bodyshapes of pros for clues to the bodies a sport creates.

A good physical yoga can give you a leaner yet muscular physique... along with so many other good benefits. It's all body weight work.

Running has added mass to my legs so I would avoid that. Just reduce calorie intake and lift light and high rep.

Easy..Endurance training*strong text* VERY high amounts of cardio, Decrease calorie intake and NO PROTEIN or post workout snack. Work whichever muscle group you want smaller with very high reps (100-150). No resistance training at all if your trying to rapidly decrease muscle size. Im telling you thus works but you must be consistent. Goodluck

Standard American diet...

Really though, working with a low protein diet, in a calorie deficit, with some steady state cardio, should do it.

Give some to me!! And if you've got an extra fat I'll take that too. I have the opposite problem and am underweight :)

I would focus on getting the majority of your calories from carbs (maybe fruit and rice) and reduce the amount of protein and fat in your diet. Do more cardio. Have you noticed how most endurance athletes are very lean?