Before you ask for a raise, the most important thing you need to remember is that you need a reason for asking for one. An employer is not just going to hand out extra money to you because they like you - you need to give them a compelling reason to do so.
Basically, you need to give them something that exemplifies your hard work and that shows you are a positive asset to the company. Think of a few ways for doing so - below are just a few examples:
Arriving on time on a consistent basis
No employer likes workers who show up late - ever. If you consistently come to work early or on time, your boss will definitely take notice of this and will appreciate your punctuality. You've already given yourself a head start.
Taking on an extra work load
Volunteering to do more than what is expected of you helps to build your reputation within the company. You will be recognized as a leader, as someone willing to help out and as someone who can be counted on. You may also gain some valuable experience within other departments, and extra knowledge never hurts. Employers love this type of employee, and will be more likely to go the extra mile to keep them on board.
Keeping track of your performance
There is nothing better than being able to show concrete examples of how you have benefited the company. Have sales dramatically increased since you came on board? Do you consistently meet or exceed your targets?
Of course, some people argue that taking on an extra work load or working overtime is a negative because you allow the company to take advantage of you. Well, like it or not, this is how the world works. If you want to stay in the same position year after year, do the minimum, but if you want to move up, putting in that extra effort will be required of you. Raises are not free handouts for everyone - they are reserved for the ones who put in the extra effort.