How To Experiment w/ 2nd Template?

I’d like to experiment with a 2nd template until it’s to my liking and then move it over to replace my current template. (My current template, by original design, cannot do what I’d like it to … or so I gather from reading through the forums here.)
I know some of you must play around with new templates and eventually replace your old one. Any tips on how to experiment while leaving your current blog available?
Also, any precautions I should take on protecting my archives/data while experimenting and eventually replacing my template?
Off topic: I’ve found these forums to be VERY helpful while making the transition to WordPress from Blogger. This resource is much appreciated.