I, like you, tend to have to work to keep my weight up to normal. For type 1 diabetics as for all people, gaining weight means increasing calorie intake. However, as you know, this is complicated somewhat by the fact that we need to increase insulin doses if we increase carbohydrate intake.
For starters, if your cholesterol levels are within normal limits, you may be able to gain calories by increasing protein in your diet. Lean proteins can mean added calories without increasing insulin. Increasing the "good" fats in your diet may also be an easy solution. By "good" fats, I mean adding unsaturated fats such as safflower oil and some of the newer healthy margarines that have no trans fats in them. You may also safely increase the no-carb vegetables, such as leafy greens that have no carbohyrdates, although these kinds of foods are also low in calories and may not help with the weight gain attempts.
If these foods do not help you gain enough weight, you may need to also increase your carbohydrate intake. If you attempt to increase carbohydrates, you will need to increase your insulin dosage. If your doctor has set you up with an insulin-to-carb ratio, then you use this to know how much to increase your insulin. If not, then before adding carbohydrates to your current diet, you will need to talk with your endocrinologist to learn how to increase your insulin appropriately to cover added carbohydrates.
Another thing to look at is that your glucose control must be good in order for you to be able to keep your weight up to normal. If your sugars tend to run higher than normal, you may be burning off your body fat in ketosis. In order to keep your body fat, you must keep good glucose control. If your a1c numbers are under 7, this should not be an issue, but if your hemoglobin a1c is usually above 7, you may need to tighten up your control to lower the daily glucose levels in order to maintain good weight.
I hope this helps you. Some of us are just need to eat more than most people in order to gain weight and to keep the weight normal, but with the proper balance of calories and insulin, you should be able to reach normal weight and stay there.