I said I would follow up on this topic a couple weeks ago, so I'm going to do it today before I forget and someone gets mad at me.
I think of the weight dropping issue as two different processes: losing weight and cutting weight. Losing weight
is a long term process to permanently or semi-permanent lower your bodyweight. Cutting weight is a short-term process to temporarily reduce bodyweight in order to make weight for competition. Accordingly, the approach for each is different.
Obviously this is a huge topic and I'm not going to discuss it in great depth. I also don't feel like doing the quantity vs. quality debate (hint: you need to take care of both). Really this amounts to 1) Ensuring your diet is constructed on quality animal source protein, quality fats, and varied vegetable matter and then 2) Ensuring that you reduce your overall intake, with #1 considered, enough to cause a change in weight.
The first point is pretty simple. If you don't get it, I'm not sure what to tell you. Get that squared away, and on that foundation, build up the rest of what you need. Yes, I do think you'll lose more weight more easily by reducing your carbohydrate intake significantly. Drop it to the minimum necessary to keep yourself training well. If you've been living on Carl's Junior and Taco Bell, fixing the food quality will automatically reduce your caloric intake. However, if you've already been eating well, particularly low-ish carb, you'll have to actually pay attention to reducing caloric intake. Reductions should be as small as necessary to get the weight moving on schedule, and then reduced repeatedly in stair-step fashion as bodyweight lowers. Dramatic caloric reductions tend to simply make you feel terrible, train worse, and even sometimes lose less weight; and what you do lose will not stay off as well.
Depending on your sport, you may be able to modify your training somewhat to assist in the weight loss. For power and strength athletes, I'm not a big fan of adding conditioning work for weight loss. Get it done with nutrition and keep your training appropriate for your performance.
If you're looking for a permanent weight change, you need to get down to your target weight and maintain it for a period of time to reset your body. The more slowly you drop, the easier it will be to maintain. A serious consideration is how losing weight will affect your training. If you drop a lot of weight in a short period of time, your training will suffer without a doubt. This can be mitigated, again, by slowing the process as much as is allowed by your competition schedule. Plan ahead and get it done. Don't be the guy who emails me asking what the best way to lose 12 lbs by next Thursday is (but without any performance impact, of course), especially if you've known you had to do it for the last 6 months. This brings us to:
If you for whatever reason have to lose a good chunk of weight in a short period of time, you won't be able to simply rely on nutritional changes. In this case, you'll be relying on dropping water weight. This is not necessarily healthy (although it's not necessarily unhealthy either); just be smart about it.
The tradition method of losing water weight is of course sweating it out. Depending on how it's done, this can be extremely taxing or somewhat taxing. The less active the process, the better, i.e. relaxing in a sauna is better than running laps in a rubber suit. But even sitting in a sauna can be physically draining.
Better is tricking your body into dropping water without having to force it out with sweat. The following process is something I learned from Robb Wolf
and a few of my lifters have use it successfully. The basic idea is to drink way more water than you need and then systematically reduce your water intake. Your body will keep flushing what it thinks is excess water even as your intake is reduced, dehydrating you enough to create some pretty significant weight loss. 5-7 days out from weigh-ins, drink 5-7 liters of water. Reduce it by about a liter a day until you're one day out - on that day, drink as little water as you can manage, and continue that until your weigh-ins.
The key to any dehydration weight cutting is that you MUST rehydrate before competition! The best time to start is the moment you get off the scale: the sooner the better. You're going to find yourself making a lot of trips to the restroom during this time even though you're dehydrated, so plan on drinking more water than you actually need. 35 ounces (liquid) of water weighs a kilogram, so plan on replacing 35 ounces of water for every kilo you've lost, plus extra you'll flush (literally).
If you've cut a lot of weight, replace electrolytes with your water. The classic way is to drink Pedialyte. The cheaper and sugar-free way is to add a half-teaspoon of salt and a half-teaspoon of potassium chloride (salt substitute) to every liter of water you drink. This will take care of any lost electrolytes, and also make an isotonic solution that will be absorbed better.