I tend to have the same problem.
It's a very hard habit to break but here are some suggestions --Come up with a hardened character--don't make her seem so weak and helpless
-Try not to get into situations where the girl is hurt and needs Legolas's help or vice versa (that's usually how Mary Sues are started)
-Just have her be his friend (this is the hardest of all)
-The one That I found have works best is having two stories, one with a mary sue and the other without one. When you want to write mary sues, go on the one with her in it. This should fulfill your need and write the other one when you are mad or just not crushing.
I hope some of those help. The last one really helps me and I've actually managed to keep some of them anti-mary sue!
:):):):) It's extra hard for me becauses I write romance, gotta kill off all mary sue writing habits, I must admit that it fun to write them sometimes. *smirk* i would know I wroten my fair share of sues but they've all ended up in trash can.