How do you sync apps to an iPhone without deleting already downloaded ones?

If you have already purchased apps while online with your iphone or Touch, open iTunes with your device connected, go to File > Transfer Purchases From "your iPod."

To sync apps bought with another account, go to Store > Authorize Computer, and enter the username and passWord used to purchase those apps. Note that you can only authorize 5 computers with your account, so choose wisely.