There's not a ton of players that show up on the first day of practice asking coaches if they can play on the offensive line. When I was in high school 10 years ago, all of our lineman wanted to be lineman. We believed in ourselves. Not only did we believe in ourselves, we believed in ourselves because our teammates and coaches believed in us. This is the key to building mentally-strong lineman, which could in turn ultimately result in more wins.
From the time I first started playing Pop Warner Football, I have been told football is won on the line. In my high school, the offensive line would get an extra 30 minutes of individual drills while the rest of the team would only get 15 minutes of offensive individual time. I will never forget one high school game where we had a rough start. The offensive line was missing blocks that we shouldn't have missed. Our quarterback walked up to the huddle and said, "Listen up: if I go the rest of the game without getting sacked again, I'll buy everyone ice cream at school tomorrow." He didn't get sacked for the rest of the game, and we all got our ice cream.
Currently I'm an Offensive and Defensive Line Coach in a high school as well as the Head Football Coach at a middle school that plays football in the winter. I find myself in a challenging position where I'm having trouble motivating athletes to play the offensive line. Everyone wants to play a "skill position." I recently went to a football clinic with a coach from Utah as the guest speaker who said "all positions are skills positions."
Build up your linemen's mental toughness by underscoring how all positions require skill to play and all positions require different skills.
The key is to not act as if the offensive line is lesser and do not let the other kids act this way either. When you are putting kids into positions, tell them "this is where your skills fit better." In some cases, kids are good football players but may be too slow to play in the open field, so they fit better on the offensive line. No matter your reason for putting them where you put them, the Offensive Line is a "skill" position.
Another way to encourage your linemen is through awards. One coach I know gives out a "Pancake of The Week" award to whichever lineman has a great week at practice. At the end of the year, the player with the most awards receives a trophy. You can award players with decals; something as small as circles on the back of the helmets motivates players. This is a way to create a friendly competition amongst the team, with teammates counting each other's stickers and talking smack to each other.
Creating a mentally tough offensive line can be challenging depending the type of athletes you have.
The most important part about creating a good offensive line is telling them how valuable they are.
Our performance determines the flow of the game on the decisions made during the game. In many cases, the offensive line determines what playbook you should run. The bottom line is that every team needs to encourage and believe in their offensive linemen to help build up their confidence and mental toughness.