With the unsteady economy, rising unemployment, and companies downsizing, everyone is doing all they can to make sure that they are not going to lose their jobs. One thing that can make a boss feel you are replaceable is being unavailable if they make a reasonable request for your help on an assignment in the terms of overtime.
Yes, you may have plans or do not feel like working overtime, but if you do not someone else will be willing to work that overtime and they are the ones who will be impressing the boss. If you know that there is a project or the opportunity for overtime, volunteer to help out or work. This will look favorable when it comes to raises, promotions, or just keeping your job. Just do not make the mistake of over extending yourself and not getting your regular work done.
No matter what type of job you are working at, if you have any questions do not be afraid to ask them. If you are unsure how to do something but just decide to do it the way you think it should be done, it could be wrong and have to be redone. Not only did you waste man hours doing it wrong but the whole project may need to be scrapped. This will cost the company money for the scrapped project but also money to do the project the right way. This will surely get the bosses attention but not in a positive way.
Remember these couple of little mistakes you want to avoid to help keep your job.