How would you diplomatically remind your boss at your job to approve your employee mileage reimbursement?

My boss has fallen behind few months behind in approving my mileage reimbursement. This had happened a few times in the past. He even apologized in the past. This time, he has fallen behind 4 months. The amount is not very large, but still matters to small employees like myself. I feel like I'm in a bind. ... show more My boss has fallen behind few months behind in approving my mileage reimbursement. This had happened a few times in the past. He even apologized in the past. This time, he has fallen behind 4 months. The amount is not very large, but still matters to small employees like myself. I feel like I'm in a bind. I must try to maintain a good relationship with my boss, and yet I need my mileage reimbursement. If this gets resolved this time, the chances are it will happen again down the road. How would you recommend that I approach this issue diplomatically w/o getting on my boss' nerve? Your advises are appreciated.
Update: To all who responded, I've tried email reminder in the past, but why do I have to keep reminding him? It's not that I feel bad about asking for my money. I don't want him to get any impression that I'm pestering him or nagging him. If he does, it can affect bigger amount of money down the road in... show more To all who responded, I've tried email reminder in the past, but why do I have to keep reminding him? It's not that I feel bad about asking for my money. I don't want him to get any impression that I'm pestering him or nagging him. If he does, it can affect bigger amount of money down the road in case the business relationship turn wrong way. Are there anyone in management at their job on this board? I'd like to get the opinion on from the other side of the table from the managers.
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