It gets very hot in Burma, so don't over pack your suitcase: fill it mainly with light clothing and the essential things you need to protect your skin from the sun. If you forget your sunscreen, you can always try thanaka: a yellowish-white paste of vegetable origin used by the Burmese (especially the women) to protect themselves from the sun.
It can also get quite cold in some areas, especially Pwin Oo Lwin, Kalaw and Inle Lake. Temperatures fall to around 5 °C to 10 °C at night between November and February. It is therefore advisable to take some items of warm clothing, especially if you are staying around Inle Lake, where most of the hotels stand on stilts.
When visiting Burma, it is especially important to dress appropriately for the local culture: women should avoid shorts, sleeveless tops and short dresses. If you are wearing a short-sleeved T-shirt when about to enter a pagoda, you should place a piece of cloth over your shoulders. Nobody will comment if you don't, but it may make people you speak to feel uncomfortable. One possibility is to buy a traditional Burmese "longyi": a kind of wraparound skirt worn by both women and men. These offer the advantage of being both practical and cooler than trousers.
Consider taking a good quality camera with you: the scenery in Burma is wonderful, and you won't want your photos of Bagan and Inle Lake spoiled. It is difficult to buy electronic equipment in Burma (except in Yangon perhaps).
Finally, don't forget to take all your essential toiletry products. You can easily find soap and shampoo in Burma, but this is not true of the other products you normally use. And the fact that most moisturizing creams in Asia contain skin whitening agents makes it all the more important to take your own. Something else to be aware of is that it is impossible to find tampons in Burma. Sanitary pads are available however.