1. Think of Lent as a weight loss program. "I'll give up sweets and try and knock off a few pounds." Uh, dude: missing the point.
2. Doing nothing. "Yeah, Lent. I'll think about that. I'm not sure what I'm going to do yet." And then it's Easter....and your relationship with God hasn't changed one bit.
3. Do too much. "I'm going to fast between meals every day, join a Bible study, give up Diet Coke, make handmade Easter gifts for everyone I know..." Yeah, again, focusing on yourself and not God. Plus doing too much = surefire fail.
4. Measure yourself against others. "Geez, he's giving up THAT? Maybe I should...." Your walk with Christ is completely different from anyone else's.
5. Not listening to sound spiritual advice. One Lent, I was going through a very difficult period in our family life. I was planning my Lent, and my spiritual advisor said, "Don't give up anything. You've given up enough." Thankfully, I did listen.
6. Giving up. Lent is a marathon, not a sprint. Run the entire race.
7. Making sure everyone around you knows exactly how HARD you are working this Lent, how MUCH you're giving up. Yeah - you're seeking glory for yourself? Good luck with that.
8. Pointing out other people's perceived Lenten flaws. "Oh, you're eating chocolate? Well...I guess. I'm not eating sweets this Lent." Besides having someone want to smack you, you also don't know what anyone else is doing for Lent. That person nibbling on a Hershey bar may be walking around with a pebble in their shoe for the entirety of Lent, and not saying a word about it.
9. Forgetting that Lent is a walk with Christ and not a walk with yourself.
10. Not asking for help. We have a great cloud of witnesses, including our Blessed Mother, who have walked this walk. The saints are so willing to help us, intercede for us, light our paths. Ask.