Don't fear the blazer.  This simple addition will make your comfortable tshirt into a dressy outfit.  Its an easy way to make any outfit dressy and casual.  Make sure that your shirt is relatively free of patterns and distracting words.  In general its a good idea to have no logos or words on any clothing for an engagement shoot.

I am a personal fan of the Vest, I usually wear this without a jacket but it can be worn under a jacket with a tie or a bowtie.  Vests are great because they add style to your outfit and slim you down.  A vest can be a simple black or gray vest or it can be a style statement with a pattern or a bold color.  Just make sure you pick a color and pattern that suits your style.

A good summer look that is still cool but smart is the polo shirt.  The main rules still apply with the polo, you need a good solid color or non distracting pattern.  The polo looks great in bold colors so go outside your comfort zone a little and try a brighter tone than you would normally go to.  The polo can be worn with shorts and pants.  Avoid polos with large logos or graphics.

You really have endless options in this area and its very hard to go wrong.  The most important thing for guys is fit and comfort.  If our clothes don't fit we look disheveled and if we are uncomfortable we are unhappy, both options result in a bad look.  Your pants can be something like a good pair of jeans that go with anything or a loud color that makes your look bold.  Just pick something that you like and are comfortable with, you want to be confident to look confident.