Most hotels and hostels will have some sort of facilities to store your valuables, even in developing countries. Communication networks are well developed among travelers. Hotel owners know this and will do their best to protect their reputation in even the most isolated places. A positive or negative review in a popular travel guide can make a huge difference for a small business dependent on tourism. Governments know the importance of protecting the reputation of the tourist industry.
Most countries take traveler's complaints very seriously.
One way to encourage yourself to pack light is to pack your bags and then carry them around the block once or twice. Go grocery shopping in your backpack, sleep in it, wear it to class or out on dates. Ask yourself not whether you could carry it if necessary, but if you are likely to enjoy scenery and culture while doing it. Then go through your bags again and take out all those suddenly unimportant items. Be reasonable about the clothing you expect to need.