In my short experience in the industry I've been able to learn a few lessons from my experience. My five lessons in five years are:
Don't try to be a hero. It may seem like fun to be the guy working late nights and pulling crazy hours to be able to impress the boss, but it will get you and your company no where. Being the hero puts you in a position where you are far more responsible for projects than your team mates, and this not only will pit you aganist them, but it will hurt your cause as well. Have you ever looked at the code you wrote at 2 a.m.? It's not pretty let me tell you.
Don't ever stop learning. The second you stop learning is the beginning of your decline as both an employee and professional. Keeping up with standards proves invaluable when looking for employment and providing your customers with good working products.
In every job you hold, demand a positive learning environment. Companies that are unwilling or unable to invest in their employees aren't worth working for. Going to conferences and improving yourself not only gives your company an advantage but also allows you to learn and grow as a professional. This constant improvment and investment will pay huge dividends very quickly.
In your professional life you'll often come accross broken, outdated, and poorly written code. You may not always be able to, but when you can, "don't live with the broken windows.". Broken windows grow, and eventually a complete refactor will become necessary. If you can patch up the windows as you move along, these refactors become much less painful.
You're going to make mistakes, get over yourself and move on. It's just going to happen and that's okay. Mistakes are inneveitable, but always learn from them. Learn how to be a better developer.