Well you got some great answers. On what to save, you better save almost 20%. Social Security will be 13.3%. Legally he is required to file a Form 1099 if he pays you more than $600 for the year. The logbook is what will be your downfall. If I understand correctly, if you work a parts counter job on the weekend, that has to be logged as on duty - not driving.
If you are being paid with a check that causes a paper trail, directly back to you. Maybe to your benefit to go to the bank it is drawn on and cash out instead of depositing it in your account. I worked part time for a man moving equipment on the weekends during the housing boom and with my state taxes I held back 33% to cover all withholding as a buffer. Good thing at the time all my POV miles and expenses were tax deductible inside the IRS laws, I am not sure about the laws in re guards to the deductions now but you may want to check and see what you can deduct for expenses.
Also forgot one thing, if you are contracted on a 1099 who pays for workman's compensation in your state? I know most situations as 1099 you must carry workman's comp and not the originator, so in most places if you are being paid a 1099 and have a job related injury the bill is on you. Like I said, I am not sure about the state laws where you work.
Thanks Dave, I thought it was a nationwide deal but not being a lawyer did not want to say it without knowing for sure.
That extra cash is a good gig but don't tell JB. They're hardcore and they'll expect you to log it and give copies of those logs to them. Don't forget- if you do get into an accident and are technically out of hours because of a second driving job, you're screwed and could be in serious trouble. And there goes your career AND second job. Or if you get violations or out-of-service orders on the weekend. Just be aware of the risks, and the penalties.