I used to suck at time-management. I have a real grasshopper mind, that jumps from one topic to another at such a rate that few people can keep up with me and they are flummoxed by the connections that I make between topics. That said, I have come up with a few tricks that allow me to manage my time so that I actually get things done. I still sometimes miss deadlines, but that's more due to workload than lack of time management.
1) Keeping a To-Do list. The most effective To-Do list manager I have ever had is a text file on my desktop called ToDo.txt. Each task is one line. When I complete a task, I delete it from ToDo.txt. That's it. No due-dates, no priorities, no having completed tasks hanging around with when it was completed. I put everything I need to do in this list. Even small tasks, like sending an email to someone, get added to the list if I can't do it straight away.
2) Doing one thing at a time. This is hard to do these days, as distractions are just a mouse click away. But I've lost so much time by starting one task, then another task, and not getting either of them done. So I try to get one specific task done, then start on the next. If something pops up during my pursuit of one task, then I put it on my To-Do list.
3) Get one task done first thing. I try to complete one small task when I arrive at work. This gets me into the mindset of working, and it gives me a psychological boost - no matter what distractions come my way the rest of the day, at least I've gotten that one thing done. What usually happens, though, is I do that one small thing, then another, then another, and before I know it, I've gotten a lot of things done. This ties in with rule #2, doing one thing at a time.