(This post is for both freelance and agency represented models but I'm writing this mainly to address freelance models since they have the challenge of representing themselves.)
Modeling is a business and in order to be successful, you'll have to be responsible for making your career run as smoothly as possible. In my time and experience, I have come to rely on the following items to help me stay as organized as possible (I may update this list from time to time if needed):
Day Planner: Whether you have a fancy PDA or go with the old school hard copy day planner (like me), you will need to have your schedule organized. Be sure to mark down all events related to modeling such as castings, shoots and meetings with photographers, stylists, etc. You'll come to find yourself living by your day planner--having events recorded in this manner will keep you from double booking and also help you plan your agenda with stuff that's not related to modeling. Because I'm super organized and a bit of a nerd, I also have a wall calendar that I record everything on so I'm covered both ways. It also helps to color code each event so that you don't get confused. For example, I circle photoshoots in blue, box meetings in red, and circle castings and go-sees in yellow. If I know I'm going to get paid on the same day as a shoot, I'll box the event in green.
Stack of Printed Headshots: Not only does submitting yourself for modeling work include emailing a digital copy of your headshot but having a hard copy print as well. It is essential that you have a small pile of headshots printed out and ready to go when you are. Never go to a casting or go-see without your headshot--even if they end up not taking it, it is always better to bring one just in case. Be sure the photo reflects how you look now and not years ago. Remember, if you've changed anything drastically like your hairstyle or hair color, retake your headshot. Color is best...black and white isn't really in demand for headshots these days. You don't have to pay a ton of money to get your photos professionally printed. If you have a good photo printer at home you can easily print them out yourself and save a lot of money. Be sure to use photo paper with either matte or semi-gloss--you don't want your headshot to be super shiny. You'll find yourself going through your headshot pile pretty fast if you have a lot of castings scheduled so always be prepared. 8"x10" is ideal.
Stack of Printed Resumes: If your resume is only one page you can simply print it onto the back of your headshot. This is a common practice in the industry. If your resume is longer than one page, print it out separately and staple it to the back of your headshot so that it doesn't get separated when you turn them in to the casting office. Don't worry about putting your resume on fancy stationary or putting your picture on it since it will be with your headshot anyway. Keep the font easy to read and avoid font that is too large or too tiny. Always pair your headshot wth a resume when you go in for a casting or go-see. You can use your regular computer printer and plain computer paper to print your resume out on. Don't forget to update your resume as you book work and only turn in the most recent version. Be sure to put your contact info on your resume as well (email address and phone number).
Portfolio: I'm referring to the actual binder/case that you place your hard copy modeling photos in. Not all castings require a portfolio to be shown but it never hurts to bring it with you, especially since most casting people will want to take a look if they see you have one.
Comp/Zed Card: You'll want to have a good quality comp/zed card both in digital and printed format. Remember to stick to the standard sizes for comp cards (no larger than 5.5" x 8.5") and have a small stack of them printed out and ready to go as well. If you have an agent, they will supply you with your comp cards. Freelance models will have to be responsible for having their comp cards designed and printed.
Business Cards (Optional): I find having a modeling business card super helpful and I would personally and professionally suggest having one--especially if you aren't able to get comp cards done. Business cards are ideal for networking with potential clients and are easier to pass out than comp cards (unless you're at a casting of course). Basically modeling business cards are best for keeping in touch with people you've worked with or if you happen to meet someone outside of a shoot or casting environment that you want to network with. They are easy to design and have printed as well. Always carry your business cards with you, even if you're just out and about running errands--you never know who you will run into that could prove helpful to your modeling career down the line.
Makeup Products: Male or female, you'll need to have the ability to do your own makeup (and hair). This goes for castings and shoots alike. For castings and go-sees you will be responsible for your makeup anyway so be sure you know what you're doing. Women should stick to the basics: foundation, powder, lip color, gloss, mascara and eye shadow. Male models won't need the same makeup as a female model but I would encourage carrying around pressed powder in a compact if your complexion tends to get oily. Shine on your face is never good for castings. Having makeup products of your own for shoots where there is no makeup artist present will also help you look your best on matter the situation.
Email & Cell: These may be obvious but I want to make sure to add it anyway. Your email address should be businesslike and not something out of high school. Preferably having your name as your email address is best. Create a new email account if you have to so that you won't get your modeling emails confused with your other emails. Also be sure to check your spam folder regularly--I can't tell you how many times I've found important emails with details for shoots and events somehow relocated to the spam folder. When it comes to your cell phone, make sure your voicemail intro is also proper and businesslike. Be ready to answer calls from numbers you may not recognize or that are blocked--it may be a dreaded telemarketer but it could also very well be a client hoping to hire you for a modeling gig.
Notepad: I'm referring to the small ones you can easily keep in your purse, pocket or in your car. To be even more organized I like to keep a small notepad handy. I make sure to use it to write down driving directions to and from gigs, as a place to write down contact info for clients, photographers, casting directors and other people for networking, and for jotting down general notes in regards to wardrobe, parking situations and other tidbits of info that I feel I need to keep track of. So far I've filled up 2-3 such notebooks! And I don't throw them away. Many times castings take place in the same areas and so it helps to save all my driving directions that way I save paper and don't have to spend time Googling how to get from point A to point B.