There is very little need to worry about leaving something important behind when you visit Malaysia. Toiletries, medicines, clothes, film, sun tan lotion and straw hats are all readily available in most towns, and definitely in the large cities. So travel lightly. There is a post here about how to travel light for women. Read about it to find out more.
If you are planning to visit the hill stations, a light sweater would be a good idea for the cooler evenings. In more remote areas, you will not have the luxury of a shaving point, but disposable razors are widely available, or you can buy a battery-operated razor. Sanitary products for women are also available, but tampons are not easily obtained in small towns.
In Malaysia’s tropical climate, think cotton. Malaysians are fairly informal but they do dress up for dinner or a night out, especially in the cities. In fact, the fancier establishments have dress code. Moreover, since this is a predominantly Muslim and conservative country, observance of local customs is important. T-shirts and longish shorts are fine, except in rural areas and especially if you go off the beaten tourist track, in which case, keep your legs and upper arms covered. Check the required attire before entering any house of religious worship.
Cheap clothes are everywhere such as colorful batik shirts and “I was here” T-shirts make good souvenirs, too. Large shoe sizes could be a problem though.
Toilet paper is still not widely available, even in cities, so unless you are familiar with the local pail or house-and-water method, pack a roll, or carry tissue paper with you.