Get the facts and figures. Look for salary surveys, articles in newspapers and magazines, even job postings from the 'Net if they are realistic. Document what sort of pay other companies are offering for similar jobs. Be sensitive to your employer's business health. If profits are down, so are your chances for a raise.
Timing can be critical.
Again, go for facts and figures. Annualize the savings your efforts have produced. Consider using past performance appraisals or letters of commendation. Be very realistic with and about yourself. If your reasons are not objective, factual and bottom-line oriented you are on shaky ground.