If you are currently in a sales position, happy, satisfied and are successful at what you are doing, then why would anyone want to look for another job while employed. Employment used to mean looking for a good company and spending your whole career at that company. I have been seeing more and more people change jobs, not because they wanted to, but because of circumstances beyond their control. This is the reason why it is important to look for a sales job while currently employed.
Now you won't want to look as hard as you would if you didn't have a sales job. Your approach is much more casual and you are basically "looking" and learning, but not pursuing. You look to see whats out there, what other companies are hiring and keeping an eye out for trends. These trends will help you see if your chosen sales industry is thriving or struggling and to help prepare you in case if affects your company or your job. Take for example, the pharmaceutical sales industry. This sales job was one of the most highly sought after sales job and the jobs were abundant as companies were hiring in droves. As trends changed, companies changed and the entire landscape of pharmaceutical sales have changed. If you would have kept your ears to the ground, you would start to see the writing on the wall and see the suttle shifts in your industry. By keeping a pulse on things, it helps you to discover a plan B, in case things don't work out with the current company you are with. You will discover other similar companies with similar roles. Look outside of your focus area into other areas that you can transfer your skills to and expand your skillsets and in the case of the pharmaceucal industry, you might want to look into B2B sales, or software sales etc.
This "looking" activity can be as casual as looking on the job posting websites or career sites of companies. Do some research and learn about new products or services that target companies are bringing out. One of the best ways to "look", is to actively network. To effectively network and keep an eye out on sales, you should book yourself and schedule time on your calendar for such activities. When I was a sales rep, I used to go to at least on sales job interview a year, just to see what other opportunities were out there and to learn about other companies as well as hone my job hunting and interviewing skills.
Don't wait until you have to find a job to start looking for your next sales job. Plant the seeds and prepare in case things happen that you couldn't forsee. If things go bad, it is always not a good feeling, but at least you will have a head start and know where to start as you actively pursue a new sales job. Set some time once a quarter on your calendar to "look' and to network.