Nothing is more important than updating your portfolio when it comes to modeling. It is wise to have a print version of your portfolio in a simple, black portfolio case. Online portfolios are good, too, and easier to put together but having a hard copy portfolio is crucial when it comes to meeting with clients face-to-face. Your portfolio should have between 10-15 of your best images. If the image doesn't make you go "WOW" then it shouldn't go in. You should update your portfolio every other month or so but will mostly depend on how much work you get and how many images you receive.
You don't need to have an image from every single shoot you've done but you do want to make sure to change out older photos with newer, better ones. Each time you do a photoshoot, look over your pictures and if there are one or two that you think look great, then add them to your portfolio.
Don't feel the need to spend a ton on getting a portfolio put together, either. You can buy a portfolio case at an affordable price at any office supply store. Keep it basic and don't add designs or fancy stuff to it. The purpose is to show off your pictures, not the folder holding the pictures.