What are some helpful techniques for applying stickers?
For a rectangular sticker on a rectangular surface, I usually try to center it between two sides and align it along the top.  This does cause the problem of centering the sticker up and down.  

If I really care about the proper alignment, I use the end of a small knife blade to place the stickers properly.  Just stick a corner of the sticker to the tip of the blade, align and stick.  This has the added advantage that when you begin to place the sticker, the knife is still underneath and will allow you to remove it and try again if you miss.