The upgrade path of mint is a question that keeps popping up and the support of mint releases is another.
The funny thing is that any good upgrade path starts with your initial install and how you partition your disk. So actually thinking of the upgrade path before you even install is the first step. Scorp123 has done a very good partitioning howto in the tips and tricks section. Most of your upgrade problems are removed with just creating a separate /home partition.
There are two types of upgrades for mint:
Major upgrade, this would be an upgrade between major version numbers. eg Mint 3.x to Mint 4.x. KISS, backup, and just reinstall. Make sure you have a list of the extra packages you added so you can add them again once installed.
Minor upgrade, this would be an upgrade between minor or point version numbers. eg Mint 3.0 to Mint 3.1. These types of upgrades generally only have some new mint applications and artwork. so you can update your /etc/sources.list with the new mint name repository, update, upgrade and install the new mint applications. I would not worry about the artwork as it is not critical even if it is awesome.
The release notes for each edition will give more detail on upgrades.