Wikipedia article claims that there was an established linen industry in Egypt 4,000 years ago, and if they rocked this stuff on the Nile in the middle of north African summers, you have to like the chances of it keeping everyone cool during the warmer months. Whether you're ordering up made-to-measure shirts or investigating your off-the rack options, the elements of an ideal linen shirt are a little different than for cotton. Some parameters to consider:
Material: You can always opt for a cotton-linen blend instead of 100% linen, if you want a little less wrinklage. Don't kid yourself, though. It's still going to wrinkle. A lot. It'd be easier on everyone if you just accept and embrace the wrinkles.
Collars: If you aren't going to wear a jacket over your shirt, you probably want the collar to lay at least somewhat flat. This is no time to opt for wimpy little flaps. Look for a collar that is reasonably broad and not particularly stiff. And skip the collar stays, which sort of defeat the purpose.
Hem: Think about how you're going to wear this shirt. Is it going to be untucked most if not all the time? If so, consider getting a square hem instead of a curved one and a slightly shorter length.
Patterns and Colours: Linen is wrinkly, loose-woven and wavy and a bit of distressing in the print is part of the charm. Warm-weather staples like white and pale blue are of course always an option, but consider a seersucker-like stripe or an open plaid as well.