What to wear?
Layering is the byword here. Summer temperatures reach above 30°C (86°F), so light cotton or breathable clothing is advised, but even in August if you intend to head to the hills it's wise to carry a warm layer (a fleece is ideal). Spring and autumn could still be warm, so keep the light cotton outfits, but a couple of warmer layers, especially for the fresher evenings, and even one cold weather option should be part of your wardrobe. In the winter months warm and waterproof clothing is a must throughout the country but especially in the mountains.
If you are going to be doing a lot of hiking then specialist boots are advised. Ski equipment can be rented in the resort but you'll need your own ski-suit (or separate jacket and trousers) for the slopes.
The Bulgarian monasteries are not as strict as their Greek counterparts in enforcing a dress code. Shorts and vests seem not to be a problem. However, it is still wise to wear, or carry with you, items of clothing that will cover the thighs and shoulders. Remember these are supposed to be spiritual places where the demure dress of the monks should be a pointer to your own clothing.