Sports

If you train after work, don't go home first.
It's not always possible, but if you can avoid it, don't go home. The temptation of  plopping down on the couch, watching TV or being distracted on the Internet may be too hard to resist.

Try joining a training group or get a buddy. It will make you feel obliged to go.

Start small and forget your past achievements.

You may not feel it, but saying things like: "I used to run X miles in Y minutes" or "A year ago I used to press this much " de-motivates you.
After a long break you probally won't get the same results as you used to and you'll need to gradually work your way up.

Monitor your activity. You can find a wide range of apps and gadgets that counts the times you went for a run, the distances you ran, how fast you ran etc.

This is a great way for you to get excited about your workout and challenge yourself each session.

Purchase the appropriate equipment and outfits. Do not go back to your torn shoes or smelly shirts. The right equipment and clothing will give you a good feeling and help motivate you to get back on track.

Try something different.
If you don't feel like doing the same activity that you use to do, think about another that you might enjoy.