Instructions
Send your Sim off to work on time. As in real life, your Sim will have a specified time at which it needs to be at its job. Repeatedly sending your Sim to work late (or not at all) will decrease its standing with its boss, which will put it farther and farther away from promotions.
Keep your Sim happy and healthy. When your Sim returns home from work, allow it to watch television or interact with friends and things of that nature. Keep your Sim healthy by purchasing exercise equipment for its house. The happier and healthier your Sim is when it goes off for work, the better it will perform once it gets to that job.
Make sure your Sim is getting enough sleep. Send your Sim to bed every night and don't keep it up until the wee hours of the morning. A rested Sim will be happier when it wakes up, which will make it happier when it leaves for work, which will allow it to perform as well as possible at its job. The better a Sim performs at its job, the faster it will get a promotion.