1. Use social media to create your brand. LinkedIn especially seems to be a place to carefully describe what makes you of value to an employer. Link to coworkers and interest groups of career related topics. Do not use this for cat videos, political rants, or inspirational posts. In fact, do not use ANY social media for cat videos, political rants, or inspiration posts. Potential employers can and will find them. Cleanse your Facebook/Instagram/Twitter/Snapchat/Tumblr/Etc. postings of pictures from the pub crawl and delete the argument you had on Twitter about whatever was in the news. Unfriend those who blanket your feed with garbage; you need to be professional everywhere!
2. Join and attend some professional groups near you. Meetup.com is one place to find them. Establish relationships, do some educational work (give or take), take on a leadership position if you want. What counts is that others see you as a great worker with a passion for whatever field you are in.
3. A personal/blog is another way to establish yourself as a passionate participant in your field. A word of warning, you have to keep it updated, because nothing is more irritating than a stale message of your supposed competence.
4. Carry a business card with you at all times. The card can be simple: name, phone number, email, your title, and any necessary degrees or certifications you may have. Any time you leave your house, you have a chance of making a connection and you do not want to be empty handed.
Do not wait until you need a new job to prepare for the new job. It may not even require you change employers, but you always need to be up to date and ready to market yourself to anyone you may meet.