Just like you make time to keep your home clean, you need to make time to keep your portfolio clean and updated. Think of yourself as your own client and plan time to update your site with new work; keep your portfolio fresh. Keeping your portfolio current and up to date is important because you never know what could happen. You may unexpectedly get fired or that dream client might visit your site out of no where. Your portfolio is a reflection of yourself and your skills, if you have grown as a designer you should share that.
Not only should you keep your site updated but you need to keep your exposure up also. Keeping your exposure up goes further than just updating your own portfolio on your website. You should also submit your websites to galleries, post videos on YouTube or Vimeo and post your work on behance and deviantArt. The more opportunities people have to see your work the more likely you are to get noticed and land the dream job or client.