QuickBooks is definitely the way to go. If you plan on using Clio as law practice management software, you may want to get the desktop version of QuickBooks. Clio appears to integrate better with the desktop version of QuickBooks than with the cloud-based, SaaS version. (See also the section of this Toolkit on Technology for additional discussion of practice management software systems’ internal accounting programs.)
I know of some attorneys who keep track of their expenses in an Excel spreadsheet. But since most accountants know and use QuickBooks, I think it is well worth the cost of a license. Additionally, if and when you decide to farm out your accounting, it will probably be easier to do so if you use QuickBooks than if you use an Excel spreadsheet.