Do your research: are you sending your work to an established publisher or a trustworthy agent? Is the publication you're submitting work to known for treating submissions fairly? Looking for some background information will quickly tell you if you should worry about sending a manuscript to a particular publisher, publication or agent.
Avoid posting your work in forums you don't know. While there are many forums filled with writers who will give you honest feedback and will not try to steal your work, there are some sites where this is not the case. Again, some research will help you to find reputable and secure forums.
Add a creative commons license to your blog. If you are writing online, a creative content license will tell readers how they can use the work: for example, if you don't mind someone re-posting your writing as long as they give you credit, a creative commons license can allow for this.
Submit work through an online submission manager, if possible. Many magazines now use submission managers, such as Submishmash or Submission Manager, to accept work. This allows you to avoid emailing work or sending it through postal mail.
Protect your files with a password. If you are worried someone will hack into or steal your computer, you can secure your files with a password. This is not very practical for submitting to agents and publishers, however, because they will likely see this as a nuisance, so make sure to send an unlocked file or hard copy.
Read any terms of service or contract you have to sign to send work or have it published. Know what rights you're giving someone, and what you're keeping. Most magazines ask for "First North American Serial Rights," which means they have the right to publish your work, but you retain ownership of it.