Construct a Plan - Depending on what ever your goals might be whether to add lean mass or to preserve what lean mass you have while stripping away all of the fat you have to have a logical plan. The Internet is full of information along with books and magazines. Educate yourself and don't be afraid to experiment. Have a picture in your mind of what you want your physique to look like and remember that each day you can take a step towards that. Every rep of every set of every exercise will count towards your goals.
Nutrition - Something that I should have payed more attention to in my earlier stages of building my physique is the importance of proper nutrition. Look at nutrition as your fuel and building blocks. If you don't have proper nutrition and perfect it over time then it's like trying to build a house with no materials.
Monitor progress - Take progress pictures and body measurements to make sure your heading in the right direction with what you have planned out. Don't rely on the scale or the mirror or what your friends might say. All of that shit lies but pictures don't. Be real with yourself and you will know what you need to improve on when you review the pictures. I suggest taking pictures every few weeks.
Patience - One of my biggest screw ups in the past was not giving something enough time to work. Take a slow approach to everything there is no quick fix that is permanent. You have to look at every day as an opportunity to get yourself a step closer to your goals. You can make excuse after excuse but your only cheating yourself. If you want it bad enough you will have the dedication and discipline it takes to build the physique you've always wanted.
BOTTOM LINE IS TRAIN YOUR ASS OFF AND EAT THE PROPER NUTRIENTS AND THERE IS NO REASON WHY YOU CAN'T HAVE THE PHYSIQUE YOU WANT. BE PERSISTENT AND GIVE IT TIME.