The important thing is to realize that to move to testing you have to have a fully up to date stable install.
Trying it with anything less will lead to trouble.
There have been some posts of folks having trouble going from Squeeze to Wheezy lately. I have upgraded several myself with no problem.
I usually install the netinstall for squeeze and go up from there as I have the band width for it. This is a lot simpler more basic system and updates quite easily. I add things like the the entire xorg/xserver and DE after upgrading.
I think that if you leave Squeeze about as default as possible you will have fewer problems.
You need to change your /etc/apt/sources.list from calling for 'squeeze' to calling for "wheezy". That will, when Wheezy goes stable, remain Wheezy the new stable.
If you want to stay with testing at that time you want to use "testing" in the sources.list. The install I am on now started life as Squeeze when it was testing. I liked it so much that I changed the "list" to read testing instead of squeeze.
One thing to consider if you want Wheezy is there is only ONE disk you need. Disk 1. That will install the base Debian install, complete with the default packages the Debian packages with their OS. This is fully functional and there are few packages you need beyond what is on there. They are all available in the repos and you can get all you want for less band width than downloading even the second disk unless you want an awful lot of packages.
Downloading and installing Whezzy that way will probably use less band width than updating and adding to Squeeze.
You may want to look at the last few pages of this "sticky" thread from this sub forum;
Very handy in setting up your sources.list. I say the last few pages because it was started in 05. The information is a little out of date on the early pages as the repo structure has evolved some since then. The whole thing is interesting however.
No matter how you get to Wheezy install the package "apt-listbugs". This will warn you of KNOWN bugs on packages you are about to install or upgrades of packages installed. This is a very nice thing to have and can save you some trouble.