bro....... depends on what they look like........... or you want them to look like.
if your a colin farell who just dosent give a *&^% how your boots look as long as they are comfortable........... go find a good mud puddle and wade around in it all day. the water will get the leather wet and supple and then as you walk it will stretch where your feet move against it.....upside is the boot will be comfortable but ive had them fit one of my feet diferently and had a wierd shape..... comfortable but a little weird looking. another good thing is the boot will dry and still have the shape and it will happen QUICKLY
if you want to guarantee that the boots still look as good as new............... or if they are high dollar or skin ect........... put talcum powder inside to help keep you from getting blisters and go to a shoe store........ a real one not foot locker and look for leather creams cause as was mentioned they do sell them to help loosen the leather.