Only been at the company six months? No matter how much work you've done or business you've generated, you'll look presumptuous approaching your boss for a raise. Be honest with yourself — do you really deserve a raise? Have you taken on more than your job description calls for? Unless your position has radically changed in a short time frame, wait until your next performance review to lobby for a raise.
There's a difference between thinking you're underpaid and actually being underpaid. Find out what your job commands at similar firms. (Ask around or check out research sites like Salary.com, Payscale.com, or SalaryExpert.com.) Be reasonable and realistic or risk alienating your boss.