This term is derived from the Coronation street character Liz McDonald, who frequents tarty clothes, suitable for a younger generation, maybe 15 years ago. She wears lots of make up, and her trademark is a gold "L" around her neck.
The term "Lizzin it up" therefore means basically to put on a lot of makeup and/or dress up in a way similar to Liz. However, as of late it has been used to describe any kind of preparation before going out on the town.
Ooo Donnatella's just gone to the bog to put some slap on, she's really lizzin it up tonight!