Is your income fluctuating wildly due to side jobs or your own business?  If so, you may need (or want) to pay quarterly estimated taxes to sort of even things out and ensure that you're paying close to what you actually owe.

If it's just that you aren't having enough withheld, I'd change the form to show "married but withhold at higher single rate", each claim 0, and then have an additional amount withheld from each check. You'll have to run the numbers to figure out what that additional amount is likely to be...

Actually, my full-time job is the cause of my wildly fluctuating income. My side income is very small so far, but road construction is dependent on the weather. I had several weeks in January/February last year where I only worked 8 hours (and two weeks I didn't work at all), but I was easily hitting 60 hours per week in July and August.

Looking at my pay stubs, I'm already listed as single, so it looks like I just have to instruct them to withhold extra from each check.

If it's just that you aren't having enough withheld, I'd change the form to show "married but withhold at higher single rate", each claim 0, and then have an additional amount withheld from each check. You'll have to run the numbers to figure out what that additional amount is likely to be...

If I overestimate how much I save, then I consider the plan a forced saving plan.

But more than likely, I'll just barely have enough and get a small amount if any amount back for 2013 since I'm hoping to do well in the stock market... I may be overly optimistic though about my stock performance though...

I'm not a tax professional, but I agree with Money Beagle.  Just estimate how much tax you have to pay based off your previous year's return and compare that with what is withheld.  If it's not enough, open up a savings account and make up the difference on your own throughout the year.  This is what we're doing with our extra income, since the withholdings are mysteriously tiny when we know it will be taxed at our marginal rate.

It is crazy. I think in my case whatever algorithm is used to determine witholdings just can't cope with wildly fluctuating income like mine. Last year, I had a $100 check and a $1000 check from the same job!

I understand the logic behind simply saving the amount and trying to earn interest on it, but owing a tax bill just feels too much like debt for my taste and I'm willing to loose out on a couple dollars interest to simply withhold enough in the first place.