track mileage allowance and cash withheld for tax, how to?

[Mike Collins]

I have a small service business that involves business use of a vehicle. 
I receive cash payments for my service. I'm seeking procedural advice.

At the present time, when I receive a cash payment I do the following:

1. credit an income account for the cash payment
2. debit expenses: business miles (number of miles x mileage allowance)
3. debit expenses: tax withheld ((cash payment - mileage allowance) * 0.35)

Mileage expense goes to a personal account (as income) to track actual 
costs for fuel and maintenance.

Tax withheld is moved to an envelope, a mattress, or a checking account...

What type of account and procedure is recommended to track mileage 
allowance? I want an account that keeps track of the total mileage for 
Schedule C and an account that shows recent cash payments not yet 
transferred to the personal account. The expense account that I'm using 
only shows the total accrued amount transferred from cash. I realize I 
can use a report. If that's recommended, perhaps you can elaborate on 
the type of report to use.

What type of account and procedure do you recommend to track tax 
withheld? I want to put aside the 35% of cash received after mileage and 
other expenses. Like the item above I'd like to know how much of the 
cash I've received as income today needs to go into the pile.

Ideas?