When I use to compete and I started to drop wieight I made sure to try and give myself enough time to do it slow. Also, depending on what you have available, some gear will assist you more then others in holding onto muscle mass. I think Primobolan is a great drug at 600-800mg per week to hold onto muscle while you start to cut your calories severly. As for diet, I use to mix up my carb and fat ratios so I had the following:
Everyday I ate high protein.
I would rotate carbs in a high carb day, med carb day and low carb day. When I had a high carb day, I would try to eat as little fat as possible, when I had a medimum carb day I would eat some fat, when I had a low carb day, I would try to have no carbs and have higher fat. I would supplement my fat intake by using MCT Oil so I got the calories I wanted and had some fuel for energy but didn't eat any bad fats. It worked for me and I would do this until I started to get into the final few weeks before a show and eventually did my carb depletion and everytihg else that you do going into a show. I tried doing High fat and high protein exclusively and I didn't do well with it, but I know many people that did like doing high protein and high fat with no carbs...