I take a more hands on approach than most of the responses so far: When I add a job to my spreadsheet, I put the expected pay date and the payment method. I have 3 sheets in my exel workbook to track jobs: "To Do", "Payment Pending" and "Paid". When a job is paid I replace the expected payment date with the actual payment date, and cut the whole row out of "Payment Pending" sheet and insert it into the "Paid" sheet.
If I want to know how long a company usually takes to pay, I just check my "Paid" sheet and sort by that MSC name. I mostly know when and how the companies I work with all the time pay, so that's easy to fill in. But if it's a new (to me) company, I'll look at the thread that Mary referenced above and put in an expected payment date based on that sheet if I can't find the payment terms easily on the MSC's site.
My spreadsheet is very in depth: I have formulas to calculate the fees I've earned this year (and earned but not been paid yet, and haven't earned but have scheduled, etc.). I also track mileage on this sheet, so I have formulas that take into account the mileage deduction for each job so that I can see what my approximate net income for the year is (I don't bother adding in other expenses besides mileage since I don't have many, but if I want to add it in the future, it could be easily done... I do track those expenses on... you guessed it! Another sheet on the spreadsheet). The mileage calculations are good because I can enter the job info including payment amount and mileage to get a ballpark idea of what my net would be before I take a job if I want.
Everyone's got their own method, but I like being able to look and see "Oh, I've got about $200 that should hit Paypal this week, cool" (By sorting the "Payment Pending" Sheet in ascending order by expected payment date). But I guarantee that there are some people who would find my method insane, and I'd be just as unhappy with their's. Good luck!
Shopper in California's Bay Area