1. Always have your daily task completed before going to work. Going to work with it incomplete or not even started will slow your performance gains to a crawl. Some careers have the daily task change from level to level, so pay attention to the daily requirement and keep on top of it.

Have your Sims leave for work in the desired mood. Each career has a desired mood and being in it while your Sim is at work will give you a boost to work performance. That said, it is difficult to obtain a moodlet that lasts for your entire shift. The computer can be used to gain a 4 hour focused, inspired or playful moodlet. Brushing your teeth at a sink will give you a confident moodlet, and taking a brisk shower is the easiest way to gain an energized moodlet.

Doing these actions immediately before leaving for work will maximize the time your Sim is in the desired mood. It is also important to make sure no moods are in the yellow or red when your Sim leaves for work. Once your desired moodlet wears off, having your Sim be happy is almost is good. Heck, even ‘fine’ doesn’t hurt. But if they switch over to a tense or uncomfortable, you will notice their work performance stall out.
As an important side note: Sims who have emotional traits that happen to match the desired emotion for their career are at a slight advantage. There is always a chance that your Sim’s emotional trait will randomly result in a moodlet while they are still at work, keeping them in the desired mood longer than they would have been otherwise. There is no way to control this, but it is a nice bonus when it happens.
3. When your Sim is at work, make your default work action be “Work Hard” or (If going to work confident) “Schmooze With Boss”. Both will increase your rate of work performance gain. Doing this can shave a day off the time it takes to get a promotion, so make sure you change their work stance over to working hard every day as soon as they show up at work.
4. The 2,000 point reward perk “Entrepreneurial” is very important. A Sim possessing it will get a further bonus to their rate of performance gain. If your main goal is to have a Sim reach the top of a career, I would advise investing in this perk as soon as you can afford it.