Shirts are one of the most important elements of the well-dressed man's wardrobe. For any conceivable occasion, there is almost guaranteed to be a suitable shirt. As with blazers, thicker fabrics such as Oxford and denim make a more casual impression, and therefore pair flawlessly with casual blazers in tweed, flannel, linen, velvet and corduroy, for example. By the same token, these fabrics are an excellent way to add a casual accent to more formal ensembles.
If you plan on wearing a neck tie, pay attention to the size and fit of the collar. An overly large collar, for example, is particularly conspicuous when fully buttoned. On the other hand, if you prefer not to fully button your shirt, a smaller size can lend the open collar a more pleasing shape.
Again, as with blazers, it is important to find the right sleeve length. If the sleeves are too long, your shirt will look oversized. As a rule of thumb, shirt sleeves should extend 1-2cm from the end of your blazer.