Definitely do not over pack, especially if you're using a backpack, you don't want to have to carry it all around with you and you will be doing a lot of walking! Also if you're backpacking and travelling to "cheaper" countries there's a very good chance that clothing, underwear, equipment, medicine, toiletries etc will be ridiculously cheap compared to your home country so it makes sense to just pick them up as you go!
RE: 5 MISTAKES TO AVOID WHILE TRAVELLING