Whatever diet method you choose, there are some tips that apply across the board. (LO is my shorthand way of typing "low calorie, low fat or low carbohydrate, depending on the diet regime you are following".)

Set a date to start and clean out* all the fattening snacks, ready meals and high calorie products like mayonnaise, burgers and sausages beforehand.
*Give or throw them away. DON'T eat them!
Buy in LO substitutes for products you eat regularly, also a selection of low/no calorie dressings and dips, and plenty of fresh fruits and vegetables, including all your favourites.
Throw out the deep fat fryer (or consign it to the attic/cellar for the duration) and replace it with a lean-machine type grill.
Join a Slimming Club or set an informal one up with a few friends. If you don't want to meet other slimmers 'in the flesh', or you can't stick to a regular meeting schedule, consider joining an online group instead. As well as helping you keep up to the mark, the chance to share successes, failures, recipes and tips on avoiding pitfalls can be very helpful in keeping you on track.
Take a good multivitamin and mineral supplement. If you have joined a slimming club or gym, ask if there are any other recommended supplements (but don't be fooled into buying expensive proprietary brands 'only available' from them).
Find one or two 'treats' that you really enjoy, that will fit into your diet. Use them as 'rewards', instead of high calorie chocolates (or whatever your favourite sin happens to be). Examples: Chopped up fresh fruit with a LO fruit yoghurt poured over; crabsticks or prawnies with a spiced LO mayo dip; Peach and kiwi smoothie made with LO yoghurt or fruit juice; crudites and LO houmous dip; sugar free jelly with fruit; meringues broken up and mixed with fresh strawberries or other soft fruit; one or two items from the weight watchers shelf at the local supermarket.
Contrary to popular belief, exercise doesn't just tighten muscles and make you hungry. It has been proved that exercise actually raises your metabolic levels for up to 48 hours. In other words, it makes you burn the food you eat more quickly! So do your best to find some energetic activity that you will enjoy, or more than one. There's plenty of choose from. Some ideas: walking, dancing, digging, rowing, skiing (on a local ski-slope, you don't have to go abroad); cycling, kayaking, aerobics, yoga, martial arts, climbing, swimming, tennis, golf.

Sign up with your local group if you can find one. Make sure to schedule time for your chosen activitie/s at least 3 times a week. If you're at a loss and you haven't visited your local Health Centre for a few years, pay them a visit - you'll be surprised at how many different things they have on offer, plus there are trained staff always available to give you advice or even design a personal training programme for you. Some gyms charge for this service, but in many local authority health centres it's included in your normal membership fee.
If you haven't got any good ones, buy a good quality set of bathroom scales, Always use the same set of scales for checking your weight loss, as different scales may not agree (If you're using a local gym regularly, you can use their scales instead of buying your own). You will also need a pair of kitchen scales for weighing portions.
Make packed lunches with LO spreads instead of butter, cottage instead of cream cheese etc. (try cottage cheese, prawn and black pepper sandwiches, yum).
Find ways to reduce calories in everyday cooking.
- Instead of frying mushrooms, cook them in a little chicken stock, then drain well.
- If you MUST fry, use a spray on oil (or non-oil frying spray if you can get it). Turn eggs over, or put the lid on to the frying pan for a short time to cook the top instead of splashing them with oil. Or poach instead of frying.
- Develop spices for curries by frying in a DRY pan until they become fragrant, then add a little water to gently soften the onions before adding the other ingredients in the usual way.
- If cooking with minced (ground) meat buy the low fat variety and cook it twice: once in plenty of boiling water for about 10 minutes; then drain off all the liquid (it will take the fat with it), add a little fresh water, adjust seasoning or add the other ingredients to continue with your recipe.