I've figured out that if I put my purse (with all my cash/debit cards/etc.) in the backseat or even the trunk of the car (any where that I can't reach) it makes going to a drive-thru for a snack impossible. And usually I'm too lazy or rushed to stop and actually go in.
It's a win-win, I save money and avoid the calorie-dump of drive-thrus!
This has decreased my pre-work (and sometimes post-work) Starbucks consumption a ton.