Write it down.
That's my one and only tip for being more productive. Yes, I know this pales in comparison to the depth of advice you'll from someone like David Allen or Shawn Blanc, but it's the best universal tip I can provide.
Writing it down takes your goals and makes them just a little more concrete since they're not just in your head. Writing it down lets you see what you should be doing and makes it harder to ignore. Writing it down lets you check it off when you're done, which is oh so satisfying. Writing it down lets you organize your goals into a list, schedule, or anything else you want.
There are plenty of other details you can dive into for your specific productivity workflow, but this idea of writing your goals down on paper, whether actual paper, a whiteboard, or a productivity app, is essential to any system you settle on.
Personally, I have a list of long term goals in a Day One journal on my iPhone that has some big goals that I want to work towards. The specific things I have to do go into Remember the Milk, and really big milestones go into my calendar. On days I really need to get a ton done, I use the app Due to set times for me to start every task I have that day. As soon as an alarm goes off, I start working in the next thing, no matter where I am in the last thing.
I've tried a bunch of different productivity styles and this is just what works for me. Doing exactly what I do probably won't work for you and that's fine. I don't think my way of getting work done is the only way. However, if you're not at least writing down your goals and tasks, you probably don't have as good a system as you think you do.