I worked retail years ago and I would probably do it again if I needed a second job because YUP, they ARE always hiring and part time hours are preferred.
Pros:
If it's a clothing store, you usually get some sweet employee discounts. Heck, most stores will offer discounts, but the clothing ones are my personal favorites! AND you will always be knowledgable of the current trends 'cause you'll see 'em first.
SOME retail stores will offer commission on top of salary. Commission is wonderful.
You'll always get to talk to people.
You'll always be on your feet/occupied which is good because it keeps you away from food and boredom eating (heh heh, how professional does it look to be crunchin' on a bag of Munchies while touching other people's merchandise?

)
Cons:
Some of the people you talk to will be VERY annoying and yell at you for things that are not your fault. Some customers will expect you to fix everything for them and when you are unable to do so, they may yell again.
Being on your feet all the time, while preventing you from eating, still gets a bit tiring.
It's boring, but really, these days, when is WORK FUN?!
The pay pretty much sucks, but if you're just looking for extra, why not?
In regards to salary, up in Virginia (5-6 years ago), I made $8.50/hr starting out and then $11.50 as an assistant manager.
Down here in Florida, most retail jobs pay $7-$9/hr if you're a regular employee and $10-$12/hr if you are any sort of shift or assistant manager.