What is the best process for code review when using GIT?

Current process:

• We have a GIT server with a master branch to which everyone commits
• Devs work off the local master mirror or a local feature branch
• Devs commit to server's master branch
• Devs request code review on last commit

Problem:

• Any bug in code review are already in master by the time it's caught.
• Worse, usually someone has burnt a few hours trying to figure out what happened...

So, we would like

• To do code review BEFORE delivery into the 'master'.
• Have a process that works with a global team (no over the shoulder reviews!)
• something that doesn't require an individual dev to be at his desk/machine to be powered up so someone else can remote in (remove human dependency, devs go home at different timezones)

We use TortoiseGIT for a visual representation of a list of files changed, diff'ing files etc. Some of us drop into a GIT shell when the GUI isn't enough, but ideally we'd like the workflow to be simple and GUI based (I want the tool to lift any burden, not my devs).