As you should know by now, not having enough cash to tip is the biggest way to ensure your car gets the worst possible parking spot when you hand your keys to the valet. Now, there's an app for that.
Parqer is a Dallas-based app (it'll hit a bunch of other markets soon enough) that lets you A) find valet services in your close proximity, and B) prepay using your credit card. You even tip via the app, and all you have to do is show the attendant the receipt on the screen, which can also serve as your stub, if you're a clutz and lose it.
Have 8:15 p.m. dinner reservations (a hot date riding shotgun in your car) and it's 8:17 and you're circling the block, freaking out? Call up Parqer, and your troubles are over.