How can I use a second computer to manage apps on my iPhone? I have an iPhone and two Macs -- an old Mac I've been using to manage what's on the phone, and a new one that I've never connected to the phone.

Now I'd like to use the new Mac to buy a couple apps and add them to my phone. (The old machine isn't available right now.)

I don't want to connect my phone to the new machine and have it sync and wipe out the apps and the music that are already on there.

Is there any way to keep what I already have and add some more items using the new Mac?

(The reason I don't just use iTunes on the phone itself is that something I want to get is so large that the phone refuses to download it over the cellular network.)
You are quite correct, you don't want to sync your iPhone to this new Mac. At the least, you'd lose any songs or movies you have loaded.

You can do what you want using the iPhone Configuration Utility (free at http://www.apple.com/support/iphone/enterprise/). Install, connect the iPhone, click on the iPhone's icon, and go to the Applications tab. It will let you install or uninstall individual apps.

Or connect your iPhone to WiFi (you could use Network Sharing from the new Mac, if no other WiFi is available) to download directly, after buying from either the iPhone or the Mac.