If you are going to ask for a raise, the context for your request is almost as important as the rationale you are going to use to justify your request. Here are a few important things to consider:
1. Don't Ambush - asking for a raise is not something that you should do casually and it's not something that you should surprise your boss with. If you plan to ask for a raise, make sure that your boss is aware that this is the intended topic of the conversation or bring up the issue during your periodic review, when your boss is probably anticipating such a request. If you ambush your boss, you risk out of hand rejection.
2. Find the Right Time - if everyone is racing against time to complete a project or working around the clock to meet a deadline, it is probably not the time to ask for a raise. Schedule your conversation in advance at a time you know is going to be less hectic. Give your boss the opportunity to listen to you calmly and at length.
3. Consider the Circumstances - if your company is struggling or times are tough in your industry, think twice about asking for a raise. An untimely request may be considered unrealistic or even position you as someone who simply doesn't get it.
4. Consider the Boss - your chances for success are higher if your boss is in a receptive mood. If when the time for your salary discussion comes you notice that your boss is agitated, under pressure or otherwise distracted, consider asking that the meeting be postponed.
If you are interested in this topic, you may also be interested to learn how to NOT ask for a raise and how to ask for ANOTHER raise.