Time management is key to be successful in any business. In my opinion, the best way to start is to set realistic expectations. Do not over-commit and under-perform just because you think it isn't wise to turn away work. In all actuality its not turning work away, it is setting clear expectations. The recipient of the expected assignment can now manage their time in order to balance their workload as well.
When I first started in the business world I over-committed all the time because I thought that is what my manager wanted - someone who was a "yes", "can do" person. Unfortunately, I would miss deadlines because of the massive workload I had set for myself - which in turn led to some "not so flattering" reviews. I talked with a few of my peers and they explained that time-management was not just agreeing to do everything and then trying to find the time to complete but managing the work load by setting expectations.
Now, I have a daily planner that I use to manage my projects. I organize it in a way that puts the "must do" assignments first, the "need to do but not immediately" second and then I fill in the available time with "at some point this needs to be done but not now" items.
Every night before I leave, I set up my schedule for the next day. Also, as soon as I arrive in the morning I check my email for any updates or added requests, adjust my day if necessary and then start chipping away at my list.
I hope this helps!