Verse 12 definitely has been confused for many years. I can't claim ultimate authority on it's theological meaning, but I do have my own thoughts based on mild research and what I know about God. I don't think this can mean that someone works to achieve their salvation. That would go against what Paul teaches elsewhere. Based on my experience today, and verse 13, I can't help but the think it means that we should strive to achieve the works God has uniquely planned and gifted us for with fear and trembling.
In my neighborhood there is a man who walks his dogs whom I see regularly. He's got a big dog and a little chihuahua, so they kind of stick out. The guy also looks cool to me. He's older so his hair is gray and balding, and his beard is almost white, but groomed well, and he's always wearing shorts and flip flops, looking really casual. He looks like the laid back kind of guy I hope to be one day. I randomly saw him at Starbucks one day, and worked up the courage to introduce myself as someone who sees him around the neighborhood. While waiting for his coffee, I learned his name and a brief summary of how long he's been in the area. He then got his drink and we parted ways. I have felt that this is the beginning of a potential way to build a relationship with someone in my neighborhood. He may be a Christian, or he may be completely off the deep end spiritually, but I think God wants me to pursue this relationship regardless for some good work He might have me do in this guy's life.
Therefore, I have started trying to think of ways to run into him beyond the chance encounter at a local store. Knowing he walks his dogs, and he lives along the route I take to drive to our clubhouse gym, I decided to start walking to the gym instead of driving. It takes all of five minutes, so no big deal. About four times a week for the last few weeks I pray and hope that he'll be out walking his dogs as I walk to the gym, but I never see him. Until today, when I saw him coming out of a house to get something out of his car. He was a little too far away to have a casual "bump in" conversation, so I would have had to call out his name to get his attention. This would have, in my opinion, completely ruined my casual "oh hey, aren't you that guy I met a few weeks ago?" plan, so I let the moment pass- and kicked myself over and over for not taking the opportunity.
I know what it's like for God to "enable" me "both to desire and work out His good purpose." I want to build this relationship, and God provided an opportunity, but I sit here "trembling" spiritually at the fact that I blew my chance.
I desire to achieve the works God has for me, and what He's enabled me for through my salvation, and I will continue to pray that God gives me more chances to follow through.