This issue has been discussed many times before. There are many reasons why a SP would want to file tax returns.
However the common mistake is quoted above, that is "one that knows how to write off as much in taxes as possible". That is not usually the issue for cash businesses with no paper trails.
If there is a required reason to file taxes, ie you want to buy a house, then the proper approach is how much income is required to get the mortgage, not how much to write off.
If you are being forced to file then the safest way is to figure out your cost of living for the year and claim that. Again it all comes down to what kind of paper trail you have; bank deposits, car payments, credit card statements etc etc.