I'm more or less where I want to be now, but there are some useful tools. I find I require a combination of exercise and dieting. There must be some motivation from yourself and possibly others. Some people can just diet, but I find I need to keep track of what I've eaten and as well as what I've burned in exercise. Doing this exactly in a home setting is impossible, but if you can get to within 10% and error on overcounting calories in and undercounting exercise, you should be alright.
There are many pieces of software to help you stay on target as you diet. Some are diaries of what you eat, others add exercise, and some try to offer psychological rewards -something I find deeply annoying. For my purposes a more than good enough tool is Perfect Diet Tracker. It came highly recommended by an underweight friend who has problems keeping her weight uses it to make sure her net energy intake is high enough (a small percentage of the population has that problem). PDT has a reasonably large library of American foods - raw and processed - and allows you to enter new ones on your own as well as easily assembly recipe counts. It is very easy to find nutritional information on the Internet. Like many programs it could be improved, but it is good enough and the OS X version, through the Mac App Store, is not expensive.
You'll need a good scale with at least a 5 gram accuracy. Scales are all over the map in price and accuracy, but I went with the $25 Eat Smart, which gets great reviews on Amazon. It's controls are logical. The keys have a cheap feeling, but for $25 I can't complain. I've checked the calibration several times over a range from a few grams to several kilogram and it is mostly accurate to the gram - very impressive for the price.
I need to have a long term goal - Perfect Diet Tracker allows you to easily set one and monitor your performance. For dieting I find having a coach is counterproductive. Eating is something that happens throughout the day and I find you need to develop some willpower and a rhythm. It would be impractical for me to involve anyone else. My goals are reasonably static, so I know what I should have for breakfast, lunch, dinner and snacks. Yes - chocolate is important for my sanity as are also occasional feasts. You need to stay in this for the long haul and depravation of the essentials tempts failure.
Some people can lose and maintain weight by controlling their diet, but I also need exercise and find I need a coach to make progress. The best solution for me has been to use a sympathetic friend who is an exercise expert and can look over results, set goals that are challenging and realistic and knows when and how to offer encouragement. Mine is a friend who is a retired professional beach and indoor volleyball player and happens to be very interested in food and exercise. She is on the other coast and the agreed upon channel of communication is sms messaging with a monthly summary of exercise times and intensities. I only use a rowing machine (it is probably not recommended to just use one exercise), but rowing is reasonably low impact and I happen to have a good unit. I've been at it five days a week for over two years and it has worked out very well. If you buy an exercise machine don't skimp - there is a lot of garbage out there and something that doesn't feel smooth and natural reduces the changes you'll stay with it.
Rowing in the basement is mind numbingly boring and the final tool I recommend is an mp3 player. In my case this is a seven year old iPod that also allows me time shift podcasts and thereby justify the hour or so I spend in the basement. Some people use music, but I've had bad luck with that as the rhythm is frequently different from the rhythm of the exercise.
As far as types of food I pretty much eat what I always have. I'm a vegetarian and don't believe in diets that dramatically alter some component of basic nutrition (like high protein or low carbohydrate diets). I just tried to put something together that allowed a slow and manageable loss - in my case about a pound a week (specifically two kilos a month). That is fairly easy to achieve without forcing yourself into hunger.
If you try to lose weight you're likely to find a different strategy works better for you. Experiment around and find something that can be sustained and, just as important, that can transition into a healthy long term behavior.
update from January 2012:
A success and some learnings: comments on the maintenance phase
My weight loss has been successful and I'm now in the maintenance phase - which is proving more challenging than the weight loss. I'm finding it necessary to stay on top of monitoring my weight, food intake and exercise. Eating out can make the journaling difficult, but the application I use now has a companion iPhone/iPad app.
Everything I've read suggests that keeping weight off, once you've been heavy, is extremely difficult with a success rate of well under ten percent two years out. So my focus is now on that more difficult task. Trying to understand the signals associated with overeating is an area I'm currently investigating. The work of Brian Wansink is fascinating - things like the impact of plating on how much we eat and numerous other factors that can easily add a few hundred Calories a day. Just the sort of thing that can break your maintenance phase.
Exercise is very important. Although it takes a lot of exercise to burn Calories, exercise does build muscle. During a diet some of the weight you lose is muscle. That is bad for many reasons, but from a weight loss and maintenance standpoint supporting a pound of muscle takes much more energy than a pound of fat. So a muscular body needs more Calories than a body with more fat and less muscle even if they weigh the same. If you can stay in shape your resting metabolism will be greater than if you don't and weight loss and maintenance will be easier.
And for those of you looking to lose weight in the first place - a pointer to a useful application that will make modern recommendations based on your height, weight, sex, age and activity levels as well as your goals. It uses a more robust model than what you've probably seen. Good luck and when you make your goal remember the adjustment you will have to make to maintain your weight - probably 100 to 200 Calories a day less than what is recommended for a person of your height, weight, sex and age who has never been heavy before. This may be your best bet for staying away from yo-yo dieting.