If you have been in a job for a while and have not recently had a pay rise then now may be the time to ask. Before asking for a pay rise, you should consider what you have brought to the company to make it viable for them to give you a pay rise.
Companies need to know that you are making them money in order to be able to pay you more. The best way of approaching the subject is to make a note of things that you have done, for example, won new contracts, sold additional products and services or cut down on their overheads. In certain jobs where you are not directly involved in sales or accounts this can be difficult, but if you have put in extra hours on a regular basis or have improved a process that has in turn saved them money or time then you could also mention this.