1) Offseason focus: Maintain, don't gain
Rather than focus on bulking up, or adding strength and speed, focus on maintaining your current level of strength and speed instead. The offseason is all about improving, staying healthy and getting ready for Sundays. Improving on the little things is what really makes a difference, though. It's easier to set attainable goals like running better routes, as opposed to chasing something harder to quantify like getting stronger.
2) Skill work: Become a jack of all trades
The one-dimensional athlete is obsolete. Professional players need to be versatile. The more you can do, the more valuable you are as a teammate. It's important to find a niche, but to also improve on your areas of weakness.
3) Build a tough mentality
In order to survive at this level, you absolutely must have a tough mindset. You need to be able to understand that the next day will come when things don't go your way. Be prepared to endure the ups and downs. Don't get too high during the highs, and don't get too low in the lows. No one's path to the top is ever easy. You'll stumble. Just make sure you always get back up.
4) Hate to lose
The number one thing I can say is to always try to win everything you do. If you want to succeed at the next level, you have to practice as hard as you play. Try to win every drill, and that will translate to you winning more individual matchups on the field when it counts.