Mentally Rehearse: The following is a mental exercise that you can do throughout the day to prepare for this situation, as you are sitting around at the end of the day, driving, or commuting home. Don't focus on the workout. Instead, break down the experience into a series of small steps which are easy to do. As you rehearse the sequence of these, over and over, you become more likely to follow the same pattern when you need to do so.
Prepare by putting on comfortable shoes and clothes, and having your exercise equipment handy. You're not thinking about exerting yourself - just getting out of work clothes and into clothes that are comfortable but could be used for your workout.
1. Coffee? As you are dressing, brew some coffee, tea, or have a diet drink, etc. Caffeine (if you have no problems with it) will wake up the central nervous system, making exercise feel better.
2. Eat a snack. Low blood sugar is the most common reason for low motivation in the afternoon. An easy-to-digest snack will raise the level so that you feel good and will reduce the negative messages received from the stress-monitoring part of the brain (the left brain).
3. Weather? Just walk out the front door to see what the weather is like.
4. If you are using exercise equipment in a health club, check the availability and walk around the equipment until the user has finished his or her workout.
5. If outside, walk to the end of the block to get your bearings. Once you have walked 100 steps you are almost certain to continue. If using a treadmill, commit to walking 100 steps, then commit to running 100 steps.
There is a principle of lazy physics here: "A body on the couch wants to stay on the couch. But once a body is in motion it wants to stay in motion."