(I tried to do this as a Top 10 list but it wouldn't let me for some reason)
It's dead easy to find hundreds of tips for losing weight but, believe it or not, there are actually some people out there who need to gain weight and tips for them are harder to come by so these are my suggestions:
1) As a general rule of thumb, you need to eat an additional 500kcals a day to gain 0.5kg a week though you may need to increase this as you get nearer to your target weight. A healthy target to aim for is somewhere between BMI 20 and 25.
2) It is often easier to eat little and often, spreading your food out between 3 meals and 3 snacks during the day.
3) Nuts and seeds are healthy and calorie dense so good for snacking on.
4) Try and avoid low-fat and diet foods. It sounds obvious but it's sometimes harder than you'd think to find full-fat items like yoghurt and cheese.
5) If you're veggie, quorn is a common meat substitute but is comparatively low in calories. Try to have a cheesy sauce with it or use lentils and beans instead.
6) If your BMI is under 17.5, you might be able to ask your doctor to prescribe you with supplement drinks such as Fortisip or Ensure. These come ready to drink in bottles with a variety of flavours to choose from and are an easy way of adding calories to your diet.
7) You can also buy supplement drinks like Complan from supermarkets or pharmacies. These are usually powders that you mix with milk. "Diet" forms like Slimfast can be effective too so long as you have them in addition to usual diet rather than as a substitute. With all of these, use whole milk rather than semi or skimmed to get the maximum benefit.
8) Try putting a little vegetable oil or spread on veg.
9) Keep warm. Your body can use up a lot of energy trying to keep you at a good temperature in cold weather so wrap up warmly to help it out.
10) Some exercise is essential for good health but don't overdo it. When you do exercise, make sure you have a little extra to eat to compensate for the calories burned.
*BMI (Body Mass Index) is calculated by taking your weight in kilos and dividing it by your height in metres squared e.g. a person who weighs 70kg and is 175cm tall has a BMI of 70/(1.75)^2 which is 22.85.