To gain weight, eat nutritious foods that are rich in calories. Examples are whole-grain breads, vegetables like avocados and potatoes, lean red meat, kidney beans, poultry and also fish. Avoid skipping some meals and increase your normal portion size. Read more at http://www.ehow.com/how_3557_gain-weight.html.
Use a lot of monounsaturated fat on everything. Examples: olives and olive oil, canola oil, nuts and nut oils, and avocados. Dried fruits can add up w/ less volume. Drink juice for more calories. Eat dark chocolate when you want a sweet. These are all "healthy" high calorie foods.