By guest contributor Gloria Park Perin

Optimism can help us look toward the future, but unreasonable optimism can be your worst enemy when you’re setting goals for exercise. If you start by reaching too high, falling short of your initial goal may ultimately serve to make you feel discouraged, rather than inspired to exercise.  Even worse, overdoing it can bring harm or injury to your body.

Take stock of your life. If you’re already working full-time, raising two children, and taking night classes, then aspiring to exercise for three hours every day is probably unrealistic.  Ask yourself: Based on what my life looks like right now, what is a goal that seems manageable and attainable?

Give yourself permission to be human. We all fall off the wagon, sometimes three or four times, when we’re trying to adopt a good habit (or break a bad habit).  Be disciplined and make plans for following through on your schedule, but if an emergency comes up, don’t beat yourself up. Sit down, take a breath, and create an alternate plan, which can be as simple as promising to take the stairs to your 14^th floor office in the morning.

Be flexible. In exercise, think None-or-Something, rather than default to the All-or-Nothing thinking that can be one of the greatest barriers to physical activity. Even brief bouts of physical activity spread through the day can bring similar physical and psychological benefits. Remember this next time you want to can your workout for the day just because you can’t run the entire 6 miles.

happier.com is a personal trainer for your happiness. With more than a dozen tools and tests to help you measure, track and improve your happiness, you can trust the happier.com experts to help you reach your goals. Exclusive videos and a popular blog mean there’s something new to learn every day. Download the free iPhone application or find what you’re looking for with the Positive Psychology Practitioner Directory. happier.com is on Facebook, LinkedIn, and twitter and has meetup groups in Washington, Philadelphia, and Portland, with more planned. Click here for a social media press release from our launch.