There are four things that impact the amount of money withheld from your paycheck:
1. How much you make -- each pay period, your withholdings are calculated based on your taxable earnings (for example, if you make more one period because of a lot of overtime, it could kick you into a higher tax bracket and the percentage they withhold may go up).
2. Married versus single -- you check one or the other on your W-4, and if you ckeck single you will have more taken out than if you check married.
3. Exemptions -- the more exemptions you claim the less they withhold (line 5 on the federal W-4).
4. Additional amount -- you can indicate on your W-4 that you want an extra amount of money taken out each pay period.
You can affect your withholdings by manipulating #2, #3 and #4.