@littlek,
To give you some examples...
Let's assume you get paid bi-weekly. The federal withholding for one exemption and zero exemptions for various salaries are:
Bi-weekly pay (1 exemption) [zero exemptions]
1000 (87) [108]
1200 (114) [136]
1500 (162) [196]
1800 (230) [266]
2100 (305) [351]
If you're looking to withhold an extra $500/year you're looking for about $20 from each of 26 paycheck (again assuming biweekly pay periods). You can claim 1 single plus $20 or drop down to zero single and probably get a bit back when you file next year.