The best solution of course are these wonderful contraptions that fit in your boot.
The only problem with these are they can be pricey if you need to buy SEVERAL pairs. Now if you only have one pair of great boots then I would opt for buying a set of these. But, again if you would need to buy enough of these that you could rather invest in another new pair of boots, what I suggest is what I use, gift wrap rolls. Everyone has at least one roll of gift wrap and the nice thing is they can be cut to any size you need. The best rolls are of course the X-Lg ones at Christmas time. Just stick them in your boots and they will stand tall and proud!