I haven't had to do a press kit yet, but remembering what my theater teacher taught us about acting resumes and headshots, the number one thing was keep it current! and number two was to make it visually appealing, without being overwhelming. I would recommend maintaining a "master copy" in digital format that is ready to be emailed at a moment's notice, with a selection of pages that you can copy and paste into an email, burned to cd/dvd as needed. Keep a generic paper copy or two on hand, and print out the job-specific ones as needed, to save space or confusion later.
As far s the visual presentation, I think that having a stash of presentation folders pre-printed with your logo or generic theme image is genious, as they are pretty versatile. you may consider having a matching letterhead printed to blank pages or saved as a template in your digital format of choice so that there is continuity in your presentation. Make sure your fonts are clear and readable, save the fancy scroll fonts for headers. images should be a variety of action shots and studio stills, to give a clear representation of style and adaptability. I would put in a full-page head shot or full-body pose.
For the email version, see if your email service lets you set a simple bacground design that will enhance (but not make it too had to read) that sets it apart from a plain text email. make it very short and sweet, to-the-point, or they will just scroll through. I recommend using low to medium rersolution images in the actual message, with a link to higher resolution portfolio (you never know what someone elses' computer is capable. Please have a visible watermark or logo on your images, and disable copy/download if you are able. This is to protect the images from being misued without permission, If they need useable images for promotional material, that can be done separately.

I'm sure others will have more info directly pertaining to press kits, this is just info gleaned from my theater and photography backgrounds, but hpoefully it is a help.
Best of luck! and when `you're not sure, just smile and do what feels right.